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Participants (with Abstracts)

Blue is for talks and gray is for posters

Bainbridge, Matthew (University of Leicester), Barstow, Martin A. (University of Leicester), Reindl, Nicole (University of Leicester), Barrow, John D. (Cambridge University), Webb, John K. (University of New South Wales), Hu, Jiting (University of New South Wales), Preval, Simon P. (University of Strathclyde)
Fundamental physics from observations of white dwarf stars
Variation in fundamental constants provide an important test of theories of grand unification. Potentially, white dwarf spectra allow us to directly observe variation in fundamental constants at locations of high gravitational potential. We study hot, metal polluted white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This registers as small, but measurable shifts in the wavelengths of highly ionized Fe and Ni lines when compared to laboratory wavelengths. Measurements of these shifts were performed by Berengut et al (2013) using high-resolution STIS spectra of G191-B2B, demonstrating the validity of the method. We have extended this work by (a) using new (high precision) laboratory wavelengths, (b) refining the analysis methodology (incorporating robust techniques from previous studies towards quasars), and (c) enlarging the sample of white dwarf spectra. A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. We present preliminary results of our search of variations in the fine structure constant at high gravitational potential, from the study of white dwarf spectra.
Bakowska, Karolina (N. Copernicus Astronomica Centre, Polish Academy of Sciences)
The 2015 observations of superoutbursts in dwarf nova MN Draconis

We present results of our world-wide CCD photometric campaign dedicated to dwarf nova of SU UMa-type MN Draconis. During the 2015 June-December observing season, two superoutbursts and several normal outbursts were detected. We investigated photometric behavior of the system and derive basic parameters such as positive and negative superhump periods or supercycle length .

Barstow, Martin (University of Leicester), Casewell, S. L. (University of Leicester)
Future space missions for white dwarf studies

In recent decades, studies of white dwarfs have often relied very heavily on access to space missions to obtain imaging and spectroscopic data. In particular, they have been essential for access to wavelength ranges such as X-ray, far UV and parts of the infrared, which don’t penetrate the atmosphere sufficiently to reach terrestrial telescopes. Space missions usually take a decade or more to plan and execute. Therefore, it is important to look ahead to consider the needs of the white dwarf community and how they might fit into the future plans of the space agencies with which we need to engage. We report on recent developments and outline plans for future missions of relevance to white dwarf research.

Barstow, Martin (University of Leicester), Joyce, S. R. G. (University of Leicester), Casewell, S. L. (University of Leicester), Holberg, J. B. (Univ. Arizona), Bond, H. (Penn State Univ.), Burleigh, M. R. (University of Leicester)
Sirius B: confronting the limits of our understanding of white dwarfs
Sirius B is the visually brightest and closest of all white dwarfs and we should understand it better than any other. However, as part of a binary system, its proximity to the main sequence companion Sirius A makes it very difficult to observe from the ground. Consequently, detailed study of this white dwarf has relied on a range of space-based observatories, including ROSAT, EUVE, FUSE and HST. Photometry and spectroscopy of exquisite quality and the highest signal-to-noise have been obtained from these missions from which we have been able to study the star in great detail. In principle, the measurements made are the most precise of any white dwarf. Nevertheless, Sirius B remains a challenging object to understand. So far it has proved impossible to compute a self-consistent model atmosphere that can match observations across its full energy distribution. Furthermore, separate determinations of its mass and radius from Balmer line fitting, measurement of the gravitational redshift and astrometry of the binary remain stubbornly in significant disagreement. We examine all the systematic effects that come into play with the various models and measurements and consider what improvements need to be made to finally understand Sirius B and, by implication, many other white dwarfs.
Bauer, Evan (University of California, Santa Barbara), Bildsten, Lars (University of California, Santa Barbara), Paxton, Bill (Kavli Institute for Theoretical Physics)
Accretion and diffusion of metals on cold DB white dwarfs
Many old white dwarfs show evidence of heavy elements in their outermost atmospheres that is explained as accretion of metals from disrupted planetary bodies. These accreted metals become fully mixed in the outer convective layer of the WD, and only sediment out at the bottom of the convective zone. Hence, the microphysics of the diffusive settling must be known for these dense regions within the star, as they allow for the inference to be made of the rate of accretion for each separate element. Past modeling has inferred the accretion rates of metals onto the hydrogen atmospheres of DA type WDs, and early work has also been performed for DB type WDs. However, diffusion at the base of helium convective zones in cool (Teff<12000K) DB type WDs can take place in degenerate, strongly coupled conditions, where typical physical assumptions in diffusive calculations break down. Using recently developed methods in the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we are studying metal diffusion in degenerate, strongly coupled plasmas and immediately applying it to metal accretion on DB WDs.
Belardi, Claudia (University of Leicester), Kilic, Mukremin (University of Oklahoma), Gianninas, Alex (University of Oklahoma), Munn, Jeff (USNO, Flagstaff), Barber, Sara (Univesity of Oklahoma), Dey, Arjun (NOAO, Tucson), Stetson, Peter (Dominion Astrophysical Observatory)
Searching for planets around white dwarfs: the DECam minute cadence survey

White Dwarfs are as common as Solar type stars in our Galaxy, and are likely to host planetary systems. In particular, some of these planets may be found in the Habitable Zones of such stars. In this work, we present the first minute-cadence transit survey on a sample of 111 WDs, found in a three-square-degree field part of the CFHTLS Wide2 field. The data were collected using the Dark Energy Camera at CTIO. We obtained time-series photometry for all of the objects in our sample, and searched for eclipse-like events as well as other kinds of variability in the targeted WDs. Our search detected no planetary companions to any of the WDs in the sample. However, we discovered a new WD+dM system, in which the WD was found to be a ZZ Ceti with g = 20.64 mag and pulsation periods of 11-13 minutes. Minute cadence DECam observations of additional fields will provide stringent constraints on the frequency of planets in the white dwarf habitable zone.

Belardi, Claudia (University of Leicester), Burleigh, Matt (University of Leicester), Barstow, Martin (University of Leicester), Goad, Mike (University of Leicester), Casewell, Sarah (University of Leicester), Chaushev, Alex (University of Leicester), and the NGTS Consortium
NGTS and the search for planets around white dwarfs
The recent discovery of the disrupting planetesimals orbiting WD 1145+017 has provided us with important evidence that white dwarfs may be the hosts of planetary systems. Detecting a planet around such objects could significantly help us in understanding how systems like our own evolve after the Main Sequence stage. In addition, white dwarfs are known to have extended and long-lived habitable zones, due to their small sizes and their long cooling times. As a result, planets around them could be potential hosts for life. Transit surveys such as SuperWASP, Kepler 2, TESS, LSST and PLATO should find similar systems to WD 1145+017 and may reveal the first terrestrial worlds orbiting a white dwarf. We examine the potential of, and present the first results from white dwarfs observed by the Next Generation Transit Survey (NGTS), a new ground-based wide-field photometric exoplanet transit survey.
Bell, Keaton (University of Texas at Austin), Brown, Warren R. (Smithsonian Astrophysical Observatory), Castanheira, Barbara G. (University of Texas at Austin), Gianninas, A. (University of Oklahoma), Hermes, JJ (University of North Carolina at Chapel Hill), Kepler, S. O. (University of Oklahoma), Kilic, Mukremin (University of Oklahoma), Koester, Detlev (University of Kiel), Montgomery, M. H. (University of Texas at Austin), Winget, D. E. (University of Texas at Austin), Vanderbosch, Zach (University of Texas at Austin)
The search for extremely low mass pulsating white dwarfs

This is a status report on the ongoing campaign to detect and characterize new extremely low mass pulsating white dwarfs from McDonald Observatory. In this poster, we present the updated empirical instability strip for extremely low mass white dwarfs.

Bell, Keaton (University of Texas at Austin), Hermes, JJ (University of North Carolina at Chapel Hill), Montgomery, M. H. (University of Texas at Austin), Winget, D. E. (University of Texas at Austin)
Detection of outbursts in the coolest pulsating white dwarfs

Extensive observations from Kepler have revealed a new outburst phenomenon operating on cool pulsating DA white dwarfs. I present the observational commonalities of the presently known members of this new class of object: (1) repeated outbursts that increase the stellar flux by up to 15%, last many hours, and recur on timescales of days; (2) effective temperatures that locate them near the cool edge of the DAV instability strip; and (3) rich pulsation spectra with many modes that are observed to wander in amplitude/frequency. I also address the prospects for studying this outburst phenomenon from the ground. Possible physical mechanisms will be discussed in a complementary talk by JJ Hermes.

Bera, Prasanta (IUCAA), Bhattacharya, Dipankar (IUCAA)
Mass–radius relation of strongly magnetized white dwarfs
We study the strongly magnetized white dwarf configurations in a self-consistent manner as a progenitor of the over-luminous type-Ia supernovae. We compute static equilibria of white dwarf stars containing a strong magnetic field and present the modification of white dwarf mass–radius relation caused by the magnetic field. From a static equilibrium study, we find that a maximum white dwarf mass of about 1.9 M_Sun may be supported if the interior poloidal field is as strong as approximately 10^10 T. On the other hand if the field is purely toroidal the maximum mass can be more than 5 M_sun. All these modifications are mainly from the presence of Lorenz force. The effects of i) modification of equation of state due to Landau quantization, ii) electrostatic interaction due to ions, iii) general relativistic calculation on the stellar structure and, iv) field geometry are also considered. These strongly magnetised configurations are sensitive to magnetic instabilities where the perturbations grow at the corresponding Alfven time scales.
Bergeron, Pierre (Université de Montréal), Lecavalier-Hurtubise, Évelyne (Université de Montréal)
The impact of radiative atmospheres on spectroscopic and photometric analyses of cool white dwarfs
It has been recently suggested in the literature that convective energy transport can be seriously impeded by magnetic fields, in particular in the photosphere of white dwarf stars. We push this idea to the extreme and explore the consequences of using purely radiative atmosphere models for the spectroscopic and photometric analyses of cool (Teff < 12,000 K) DA white dwarfs. We also perform a similar analysis for known magnetic DA stars with weak sub-megagauss magnetic fields.
Blouin, Simon (Université de Montréal), Dufour, Patrick (Université de Montréal), Kowalski, Piotr (Forschungszentrum Jülich)
A new generation of cool white dwarf atmosphere models using ab initio calculations

The coolest, and thus oldest, white dwarfs observed in the solar neighbourhood and beyond represent great probes to learn more about a given stellar population through cosmochronology. However, many such stars, particularly those with strong metallic lines (DZ, DQpec) or collision-induced absorption features (ultracool WD) are currently only approximately modelled at best. The failure of current models is mostly due to the extremely high pressures encountered at their photospheres, particularly the He-rich ones. In order to fully use the potential of these stars as cosmochronometers, or access the bulk composition of the rocky material accreted at their surfaces, one must first understand the complicated physics and chemistry encountered in their atmospheres. In this context, state-of-the-art ab initio calculations appropriate for dense helium medium must be performed. We compute new collision-induced absorption spectra and non-ideal ionization potentials for heavy elements and use new equations of state and opacities appropriate to model the atmosphere of such cool white dwarfs.

Bond, Howard (Pennsylvania State University)
Hubble space telescope observations of white dwarfs in visual binaries
We will present HST astrometry of the visual binaries Sirius and Procyon, which yields dynamical masses for the WD companions, and stringent tests of WD physics, including the mass-radius relation. We will also describe new HST and ground-based observations of the strange central star of the planetary nebula EGB 6, an apparent visual binary in which the companion of the hot central white dwarf is embedded in an accretion disk formed by material captured from the PN outflow.

Boneva, Daniela (Space Research and Technology Institute, Bulgarian Academy of Sciences), Filipov, Lachezar (Space Research and Technology Institute, Bulgarian Academy of Sciences)
Flares activity and polarization states of white dwarfs in binary star systems
We investigate flares activity and emission properties of white dwarf binary stars. We apply the polarization as a mechanism to probe the flares and the released resulting radiation. The polarization could appear as patterns in these cases, as it depends mainly on the properties of radiation and geometry of the source. We employ the STOKES code partially in the calculations. The observational data of MV Lyr and γ Cas are applied. A repeated variability in the brightness could affect the degree of polarization. Detectable variations in the polarization parameters of selected binaries for the flares activity period are shown in the result. The analysis may help us to establish more evidences of the close correlation between flares, flow structure transformation around the primary star and polarization’s parameters variability.

Bonsor, Amy (Institute of Astronomy, University of Cambridge), Farihi, Jay (UCL), Wyatt, Mark (IoA), Kennedy, Grant (IoA)
The dusty accretion of polluted white dwarfs
Metal pollution in white dwarfs provides key evidence for the accretion of planetary material. The exact manner in which this accretion occurs is poorly understood. The best models suggest that the pollution is supplied by the tidal disruption of scattered asteroids (planetesimals). Infrared emission from the dusty fragments of these disrupted bodies is key to studying the accretion processes. In this work, we investigate the infrared luminosity of a sample of white dwarfs collated from the literature using Spitzer or WISE observations. Excess emission in the infrared, from dusty material accreting onto the white dwarfs, is only detected for 5% of the sample, a fraction significantly lower than the >30% expected to be polluted. We show that the observations are inconsistent with the scenario where all polluted white dwarfs accrete from physically thin, opaque dust discs, akin to Saturn's rings. We propose instead, that many polluted white dwarfs may be accreting from optically thin dusty material, that in many cases can escape detection. This theory could potentially be confirmed by JWST observations if they find an increase in the number of white dwarfs with faint excesses.

Bours, Madelon (Universidad de Valparaiso), Marsh, Tom (University of Warwick), Parsons, Steven (Universidad de Valparaiso)
Long-term eclipse timing of white dwarf binaries: a hint of a magnetic mechanism at work.

I will present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes about 70 close binaries, both detached and semi-detached and with secondary stars that are M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs. In total, we have observed close to 700 white dwarf eclipses. I use this large sample to search for orbital period variations and aim to identify the underlying cause of these variations. I will show that the probability of observing orbital period variations increases significantly with the observational baseline of the eclipse observations. In particular, binaries with baselines exceeding 10 yrs show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, binaries with late spectral type or brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits.

Braker, Ian (University of Leicester), Barstow, Martin (University of Leicester), Buckley, David (SAAO), Burleigh, Matt (University of Leicester), Casewell, Sarah (University of Leicester), Goad, Mike (University of Leicester), Holberg, Jay (University of Arizona), Lawrie, Katherine (University of Leicester)
Photometric variability and spectroscopic characterisation of white dwarfs in the Kepler 2 fields
It has long been assumed that the vast majority of white dwarfs (WDs) are photometrically stable, at the level of at least a few mmags. Alongside the well known non-radial pulsators, we have shown that 30-40% of magnetic WDs vary on their rotation periods due to star spots or magnetic dichroism (Brinkworth et al. 2013, Lawrie et al. 2016), but these constitute no more than 10% of the overall population. Significantly, however, the exquisite precision of Kepler photometry has shown that as many as 50% of all WDs may be intrinsically variable (Maoz et al 2015) but these, mainly non-magnetic, white dwarfs cannot all be explained by the effects of strong magnetic fields. Alternatively, this behaviour may be due to the accretion of metal rich material or the presence of an ultra-low mass companion. Whilst only a handful of white dwarfs were observed in the original Kepler field, the Kepler 2 (K2) mission has now observed several hundred WDs, many which have not previously been spectroscopically characterised. We discuss the initial results of our characterisation and variability study of the WDs within the K2 fields and review any potential implications.

Brooks, Jared (UC Santa Barbara), Bildsten, Lars (UC Santa Barbara, KITP) Schwab, Josiah (UC Berkeley), Quataert, Eliot (UC Berkeley)
He Star + WD binaries as progenitors to SNe Ia and AIC
White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable helium burning that is a concern for many SNe Ia scenarios. We study binaries with helium stars of mass 1.25Msun ≤ M_He ≤ 1.8Msun , which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods P_orb of several hours with non-rotating WDs. The helium stars fill their Roche lobes after exhaustion of central helium and donate helium on their thermal timescales (∼10^5 years). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass M_Ch and a core carbon ignition. More massive white dwarfs (WDs) that are primarily composed of oxygen and neon (O/Ne WDs) and lack significant carbon in their cores are expected to collapse to neutron stars (NSs) when the mass of the core nears the Chandrasekhar mass (M_Ch). When this happens from growth via accretion from a binary companion, it is called Accretion Induced Collapse (AIC). During this accretion phase, we find that the carbon in the ashes from helium burning ignites unstably near the surface. The carbon beneath the ignition point is heated until it can sustain carbon burning, leading to a inward moving carbon burning flame that is quenched when it runs out of carbon at the surface of the O/Ne core. New and smaller carbon layers are subsequently built up and burned until M_WD -> M_Ch.

Buckley, David (South African Astronomical Observatory)
New southern cataclysmic variables: discoveries from MASTER-SAAO
In this paper I report on new cataclysmic variables (CVs) discovered by the first local optical transient detection system established at the SAAO Sutherland station, namely MASTER-SAAO. The characteristics of the MASTER-SAAO system are described and the parameters of the survey compared to the Catalina Real Time Survey (CRTS). To date MASTER-SAAO has discovered over 200 (non-Solar System) optical transients with about 80% of these being likely new CVs, most being Dwarf Novae (DNe). About 50% of the DNe have outburst amplitudes in excess of 4 mag, with some extreme amplitude (> 7 mag), probable SU UMa or WZ Sge systems. The MASTER-SAAO detection limit of B = 19-20 is comparable to the ~20 mag limit of the CRTS (depending on CV colour). Based on the CV detection statistics of CRTS, we believe that MASTER-SAAO is detecting essentially the same CV population as CRTS, for a detection outburst amplitude threshold > 2.2m. I also present results of the initial follow-up program on CVs discovered by MASTER, including dwarf novae, a bright new VY Scl system and a new eclipsing polar. Finally I discuss the current and future prospects for rapid follow-up observations at SAAO of transients and other targets of opportunity, including CVs, with new robotic telescopes at SAAO, expected to become operational in the next 6 months, as well as with SALT.

Calamida, Annalisa (NOAO - AURA), Saha, A. (NOAO), Matheson, T. (NOAO), Narayan, G. (NOAO), Holberg, J. B. (University of Arizona), Axelrod, T. (Steward Observatory), Olszewski, E. (Steward Observatory), Claver, J. (NOAO), Stubbs, C. W. (Harvard University, CfA), Bohlin, R. (STScI), Deustua, S. (STScI), Rest, A. (STScI)
A network of DA white dwarfs as spectrophotometric standards for future ground-based and space surveys
I will present preliminary results of our project to establish a network of DA white dwarfs as spectrophotometric standards. The selected white dwarfs are faint enough (V > 15 mag) to be observed by future photometric ground-based and space surveys and are homogeneously distributed in the Northern and Southern hemisphere. This set of white dwarfs will have SNR ~ 200 in most of the current and future photometric surveys, such as LSST, PanSTARRS, the Dark Energy Survey, while avoiding saturation. This means they will directly allow to calibrate the data products. We derived temperatures and log(g) for our sample of DA white dwarfs by using ground-based spectra and determined their spectral energy distribution. We then used HST photometry in 6 filters from the ultra-violet to the near-infrared to set the flux scale for each star, and determined their reddening. Thus calibrated, these white dwarfs can now be used as flux standards at wavelengths well beyond the range of HST, and in any arbitrary but defined passband.

Camarota, Lawrence (University of Arizona), Holberg, Jay (University of Arizona)
DB white dwarfs in the SDSS spectroscopic sample
We investigate techniques for the estimation of distances for the large population of He-rich DB dwarfs in the SDSS spectroscopic samples. Reliable distance estimates allow determinations of interstellar reddening, luminosity and the space density of these stars.
[presented by Jay Holberg on the behalf on Lawrence Camarota]

Camisassa, Maria Eugenia (University of La Plata), Córsico, A. H. (University of La Plata), Althaus, L. G. (University of La Plata), Shibahashi, H. (University of Tokyo)
Pulsations powered by hydrogen shell burning in white dwarfs

In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarfs evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. We investigate the pulsational stability of this type of white dwarfs to see whether nonradial g-mode oscillations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up episode during the asymptotic giant branch evolution of their progenitor stars. For this purpose, we construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally pulsing and the post-asymptotic giant branch phases, and analyze their pulsational stability by solving the linear, nonadiabatic, nonradial pulsation equations for the models in the range of effective temperatures log Teff ~ 4.17-4.00, which is typical of variable DA white dwarfs (DAV or ZZ Ceti stars). Our calculations demonstrate that, for white-dwarf models with masses M < 0.7 Msun and effective temperatures log Teff ~ 4.17-4.00 that evolved from low-metallicity progenitors (Z= 0.0001, 0.0005, and 0.001), the dipole (l = 1) g_1 mode is excited mostly due to hydrogen burning through the epsilon-mechanism. The epsilon-mechanism is insufficient to drive that mode in white dwarfs evolved from solar-metallicity progenitors. We note that the e-folding time for the dipole g_1 mode excited by the epsilon-mechanism is much shorter than the stellar evolutionary timescale, hence the oscillation can reach observable amplitudes. We suggest that efforts should be made to observe the dipole g_1 mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant branch phase.

Camisassa, Maria Eugenia (University of La Plata), Althaus, L. G. (University of La Plata), Rohrmann, R.D. (ICATE-CONICET), García-Berro, E. (Universidad Politècnica de Catalunya), Córsico, A.H. (University of La Plata)

Evolution of hydrogen-deficient white dwarfs
We present full evolutionary calculations appropriate for the study of hydrogen-deficient DB white dwarfs. This is done by evolving white dwarf progenitors from the zero-age main sequence, through the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. These models experienced a late thermal pulse during the asymptotic giant branch phase and, consequently, most of their hydrogen content is burned. As a result, our white dwarfs models are born with hydrogen-deficient atmospheres. Complete evolutionary sequences are computed for a metallicity representative of the solar neighborhood (Z=0.02), considering a wide range of stellar masses. During the white dwarf cooling stage, we self-consistently compute the phase in which nuclear reactions are still important, the diffusive evolution of the elements in the outer layers and, finally, we also take into account all the relevant energy sources in the deep interior of the white dwarf, such as the release of latent heat and the release of gravitational energy due to carbon-oxygen phase separation upon crystallization. As well known, the treatment of the outer boundary conditions is a key aspect concerning the evolutionary properties of cool hydrogen-deficient white dwarfs, hence for the cooling sequences presented here, outer boundary conditions are based on a new set of improved non-gray white dwarf model atmospheres for pure helium composition (Rohrmann 2016), which include the most up-to-date physical inputs. The calculations are extended down to an effective temperature of 2500 K. Our calculations provide a homogeneous set of evolutionary cooling tracks appropriate for mass and age determinations of old DB white dwarfs and for white dwarf cosmochronology of solar metallicity Galactic populations.

Casewell, Sarah (University of Leicester), Longstaff, Emma (University of Leicester), Littlefair, Stuart (University of Sheffield), Parsons, Steven (University of Valparaiso), Marsh, Tom (University of Warwick)
White dwarf-brown dwarf binaries
There are only 5 close (period ~ few hrs) detached white dwarf - brown dwarf binaries known to date, and while these have been studied in depth, a larger sample is needed to fully investigate the effects of irradiation on the brown dwarf and the formation and evolution of the systems. In this talk I will present our new results on these known systems, including emission lines in the optical spectra of WD0137-349, the best known of these systems. I will also present the results of our multi wavelength searches for new binary systems, including confirmation of the white dwarf primary in 8 candidate systems.
Castanheira, Barbara (University of Texas at Austin), Romero, Alejandra (UFRGS-Brazil), Kim, Agnes (Penn State Worthington Scranton)
Calibrating white dwarf asteroseismic fitting techniques
The main goal of looking for intrinsic variability in stars is the unique opportunity to study their internal structure. Once we have extracted independent modes from the data, it appears to be a simple matter of comparing the period spectrum with those from theoretical model grids to learn the inner structure of that star. However, asteroseismology is much more complicated than this simple description. We must account not only for observational uncertainties in period determination, but most importantly for the limitations of the model grids, coming from the uncertainties in the constitutive physics, and of the fitting techniques. In this presentation, we will discuss results of numerical experiments where we used different independently calculated model grids (white dwarf cooling models WDEC and fully evolutionary LPCODE-PUL) and fitting techniques to fit synthetic stars. The advantage of using synthetic stars is that we know the details of their interior structure so we can assess how well our models and fitting techniques are able to recover the interior structure, as well as the stellar parameters.
Charpinet, Stéphane (IRAP / CNRS - Observatoire Midi-Pyrénées), Giammichele, N. (IRAP/CNRS), Brassard, P. (Université de Montréal), Fontaine, G. (Université de Montréal)
Signature of the core stratification in pulsating white dwarfs and tests of seismic inversion

We discuss and illustrate how the C/O stratification in the core of white dwarf stars affects their g-mode pulsation spectrum. This effect, mostly occurring through the interplay of partial wave reflection (mode trapping), leaves a subtle but detectable signature, in particular with now available ultra high precision photometry from space, that could be exploited to infer the main structures in this stratification. After briefly summarizing our current approach to this problem, we provide tests demonstrating that asteroseismology can indeed be used, in some cases, to probe the innermost composition stratification of pulsating white dwarfs. This brings interesting prospects to constrain the core chemical stratification inherited from previous evolutionary phases, which is one of the most uncertain aspect of white dwarf internal structure.

Chen, Yanhui (Chuxiong Normal University)
The dispersion of period spacing for DAV stars
Three groups of DAV star models are evolved with time-dependent element diffusion by WDEC. The core compositions of these models are directly from white dwarf models evolved by MESA, which are results of thermonuclear burning. Based on these DAV star models, we try to study the dispersion of period spacing. Because of mode trapping effect, the thickness of hydrogen atmosphere can seriously affect the deviation degree of minimal period spacings. The minimal period spacings dominate the dispersion of period spacing. The thinner the hydrogen atmosphere, basically, the more dispersive the period spacing. Standard deviations are used to analyse the dispersion of period spacing. Su et al. made a detailed mode identification on a DAV star named KUV03442+0719. Studying the dispersion of period spacing on KUV03442+0719 preliminarily, we suggest that the logarithm of H mass fraction is from -8.5 to -5.5. In addition, modes partly trapped in C/O core are found based on those DAV star models. The identified modes and average period spacings indicate that KUV03442+0719 may be the first star to 'observe' modes partly trapped in C/O core.

Chote, Paul (University of Warwick), Ashley, Richard (University of Warwick), McCormac, James (University of Warwick)
The Warwick one-metre Telescope

We are developing a robotically operated 1m telescope at the Roque de los Muchachos Observatory on La Palma. The telescope is fitted with a dual-camera instrument that allows simultaneous observations through BG40 and Z band filters, and is optimised for acquiring high-precision high-cadence photometry of variable white dwarfs, exoplanet transits, and other time-series variability. We provide an overview of the system design, current implementation progress, and early scientific results.

Chote, Paul (University of Warwick), Mukadam, A. S. (University of Washington), Aungwerojwit, A. (Naresuan University), Szkody, P. (University of Washington), Gänsicke, B. T. (University of Warwick), Sullivan, D. J. (Victoria University of Wellington)
The post-outburst pulsations of GW Librae

We present new observations of GW Librae obtained between 2012 – 2016. GW Librae was the first accreting white dwarf to be discovered with non-radial pulsations, which were wiped out in 2007 when a dwarf nova outburst heated the surface of the WD outside the instability strip. In the years that followed, we have seen pulsations return with periods near 280 and 1200 seconds, but find that their periods and amplitudes vary on timescales longer than a few hours. Some of these changes are found to correlate with changes in the mysterious 2/3/4 hour modulation that has been seen both before and after the outburst.
Clemens, Chris (University of North Carolina at Chapel Hill), Dunlap, Bart (UNC Chapel Hill), Hermes, JJ (UNC Chapel Hill)
Seismology of an ensemble of ZZ Ceti stars
I will combine the measured eigenperiods for all the hot ZZ Ceti pulsators onto one diagram. When last considered 20 years ago, the resulting distribution was highly structured. Since that time the statistics have improved five-fold and structure persists. I will compare this diagram to the output of pulsational models of ensembles of stars drawn from different distributions to see for what set of input assumptions the output distribution best matches the observations.
Cojocaru, Ruxandra (Universitat Politecnica de Catalunya), Rebassa-Mansergas, Alberto (Universitat Politecnica de Catalunya), Torres, Santiago (Universitat Politecnica de Catalunya), Garcia-Berro, Enrique (Universitat Politecnica de Catalunya)
Theoretical modeling of the population of white dwarf-main sequence binaries in the galactic disk
The companion mass ratio distribution (CMRD) of main sequence binaries is a crucial physical quantity for understanding the evolution of stars in binary systems and for constraining models of binary star formation. However, despite much work has been done during the last years, the shape of the CMRD remains rather uncertain. We present a population synthesis study of white dwarf-main sequence (WDMS) binaries in the galactic disk aimed at constraining the properties of the CMRD. To do this, we computed a set of Monte Carlo simulations of the WDMS binary population observed by the SDSS using different initial shapes of the CMRD, which take into account all known observational biases. We show that our simulations not only reproduce reasonably well the observed parameter distributions, but also we are able to constrain to some that some constraints can be placed to the properties of the CMRD.
Cojocaru, Ruxandra (Universitat Politecnica de Catalunya), Torres, Santiago (Universitat Politecnica de Catalunya), Althaus, Leandro Gabriel (Universidad Nacional de La Plata), Isern, Jordi (Institut de Ciencies de l’Espai), Garcia-Berro, Enrique (Universitat Politecnica de Catalunya)
The luminosity function of single halo white dwarfs: current status

We present a population synthesis study of the luminosity function of halo white dwarfs. By employing recent and reliable cooling sequences for metal poor progenitors and an accurate modeling of observational biases, we analyze the information that can be extracted from the currently available observational data. We find that the shape of the hot branch of the luminosity function is insensitive to the most relevant and commonly employed inputs (adopted cooling sequences, initial mass function, density profile, fraction of unresolved binaries). Also, given that the cut-off of the observed luminosity function has not yet been determined, only lower limits to the age of the halo population can be placed. Due to the reduced size of the current observational sample of halo white dwarfs, we cannot obtain definite conclusions about the effectiveness of the recently computed white dwarf cooling sequences which incorporate residual hydrogen burning.

Córsico, Alejandro Hugo (University of La Plata), Althaus, L. G. (University of La Plata), Serenelli, A. M. (ICE-CSIC/IEEC), Kepler, S. O. (Universidade Federal do Rio Grande do Sul), Jeffery, C. S. (Armagh Observatory), Corti, M. A. (Instituto Argentino de Radioastronomia)
Recent advances in the theoretical modeling of pulsating low-mass He-core white dwarfs

Many low-mass (M/Msun < 0.45) and extremely low-mass (ELM, M/Msun < 0.18-0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars (the pre-ELMV stars). In this communication, we present the recent results of a thorough theoretical study of the adiabatic and nonadiabatic pulsation properties of low-mass He-core white dwarfs and pre-white dwarfs on the basis of fully evolutionary models representative of these stars. In particular, we shall describe how the modes detected in ELMVs probe different regions of these stars and offer the possibility of applying asteroseismology. Also, we will show the location of the theoretical ELMV and pre-ELMV instability domains in the HR diagram, and how well they compare with observations. Finally, we will show the ranges of periods associated to excited modes according to our stability analysis, as compared with the periods observed in real stars.

Córsico, Alejandro Hugo (University of La Plata), Calcaferro, Leila M. (University of La Plata), Althaus, Leandro G. (University of La Plata)
Asteroseismological study of the GW Virginis stars SDSS J0349-0059 and VV 47
We present an asteroseismological study of the GW Vir stars SDSS J0349-0059 and VV 47 aimed mainly at deriving their total mass on the basis of state-of-the-art PG 1159 evolutionary models with stellar masses ranging from 0.515 to 0.741 Msun, that take into account the complete evolution of the progenitor stars. We first estimate a mean period spacing for both SDSS J0349-0059 and VV 47. By comparing the observed mean period spacing with the asymptotic period spacing and with the average of the computed period spacings, we obtain estimates of the stellar masses of these stars. We also search for the best period fit and found an asteroseismological model for SDSS J0349-0059. For VV 47, we could not find an unambiguous asteroseismological model. Once again, it is shown the potential of asteroseismology to derive stellar masses of PG 1159 stars with an unprecedented precision.

Córsico, Alejandro Hugo (University of La Plata), Battich, Tiara (University of La Plata), Althaus, L. G. (University of La Plata), Miller Bertolami, M. M. (University of La Plata)
Constraining the axion mass from a pulsating DB white dwarf star
White dwarfs are found to be good laboratories for studying the properties of fundamental particles such as the hypothetical axions. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, abandoning these stars without interacting again, and becoming an important energy sink. This would have a detectable effect on the evolutionary and pulsational properties of white dwarfs. This effect was already studied for DA white dwarfs (white dwarfs with a H-rich envelopes), but not for DB white dwarfs (white dwarfs with He-rich envelopes). We studied the general effect of axion emission on the evolution of DB white dwarfs and on the pulsational properties of DBV white dwarfs (pulsating DBs) with the aim of determining the potential of these stars to study the physics of axions. Moreover, we use an estimate of the rate of period change of the DBV white dwarf PG 1351+489 corresponding to the dominant mode to derive a new limit on the axion mass.
Cummings, Jeffrey (Johns Hopkins University), Kalirai, Jason (Space Telescope Science Institute), Tremblay, Pier-Emannuel (University of Warwick), Geislerm, Doug (Universidad de Concepcion), Ramirez-Ruiz, Enrico (University of California Santa Cruz), Mauro, Francesco (Universidad de Concepcion), Deliyannis, Constantine (Indiana University), Strader, Jay (Michigan State University)
Massive white dwarfs & the initial-final mass relation
Our group has begun to analyze the initial-final mass relation (IFMR) based on white dwarfs in star clusters. We have focused primarily on the poorly constrained high mass region (Mfinal > 1.0 Msun). This region is challenging because it is both difficult to discover and analyze these increasingly fainter high-mass white dwarfs. Additionally, higher mass white dwarfs and their inferred progenitor masses are increasingly susceptible to spectroscopic fitting errors, model errors, and cluster parameter errors. Our project has so far discovered in NGC 2099, NGC 2323, and NGC 2422 thirteen intermediate mass white dwarf cluster members and four high mass (> 1.0 Msun) white dwarf cluster members, one of these being the most massive white dwarf that has been discovered within an open cluster at 1.28 Msun. Combining these data with our uniform reanalysis of data from other groups and of the open cluster parameters has begun to limit the significant scatter previously observed in the IFMR and constrain the IFMR at increasingly higher masses.
Cunningham, Tim (University of Warwick), Wolf, Bill (UCSB), Bildsten, Lars (Kavli Institute of Theoretical Physics / UCSB)

Photoionization heating of nova ejecta by the post-outburst supersoft source
The expanding ejecta from a classical nova remains hot enough (∼104 K) to be detected in thermal radio emission for up to years after the cessation of mass loss triggered by a thermonuclear instability on the underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the hot temperatures observed in radio observations. During this same period, the unstable thermonuclear burning transitions to a prolonged period of stable burning of the remnant hydrogen-rich envelope, causing the WD to become, temporarily, a super-soft X-ray source. We show that photoionization heating of the expanding ejecta by the hot WD maintains the observed nearly constant temperature of (1−4)×104 K for up to a year before an eventual decline in temperature due to either the cessation of the supersoft phase or the onset of a predominantly adiabatic expansion. We simulate the expanding ejecta using a one-zone model as well as the Cloudy spectral synthesis code, both incorporating the time-dependent WD effective temperatures for a range of masses from 0.60 Msun to 1.10 Msun. We show that the duration of the nearly isothermal phase depends most strongly on the velocity and mass of the ejecta and that the ejecta temperature depends on the WD's effective temperature, and hence its mass.

Darnley, Matt (LJMU)
Accrete, accrete, accrete.....bang! (and repeat): a remarkable recurrent novae in M31

Classical novae are thermonuclear eruptions on the surface of accreting white dwarfs. By definition, recurrent novae are any such systems where two or more eruptions have been witnessed. The recurrents typically contain high-mass white dwarfs and are, as such, among the Type Ia Supernova progenitor candidates. In this talk I will introduce M31N 2008-12a, the single most important nova system in M31. With its one-year recurrence period, a white dwarf tantalising close to the Chandrasekhar mass, high mass accretion rate, and low ejected mass, this system is a leading pre-explosion Supernova Typa Ia progenitor candidate.

Darveau-Bernier, Antoine (Université de Montréal), Bergeron, Pierre (Université de Montréal), Lépine, Sébastien (Georgia State University)
A study of the SDSS white dwarf component in the SUPERBLINK proper motion survey

We present a model atmosphere analysis of the white dwarfs identified in the Sloan Digital Sky Survey with proper motions measured (mu > 40 mas/year) in the SUPERBLINK proper motion survey of Lépine et al. We rely on reduced proper motion diagrams to build a sample of white dwarfs in the SDSS footprint, and cross correlate this sample with the SDSS spectroscopic database to understand the systematics related to completeness, contamination, WD+M dwarf binaries, reddening, etc. We then determine a white dwarf luminosity function for this sample using various methods.

Dennihy, Erik (University of North Carolina at Chapel Hill), Debes, John (STScI), Clemens, J. Christopher (UNC Chapel Hill), et al.
Explorations of white dwarf dust disk geometry

Comprehensive observations of white dwarfs with gaseous debris have recently implied the persistence of gas in stable, eccentric orbits, as evidenced by the evolution of asymmetric double peaked emission profiles. All systems with gaseous debris also exhibit an infrared excess which is well modeled by a circular dust disk that is spatially coincident with the gas. This overlap should lead to destructive high energy collisions between the gas and dust unless their orbits are identical. We explore how relaxing the assumption that the dust is circularized to include elliptical geometries can change the disk properties inferred from the existing infrared data. We specifically apply this analysis to systems with observed gaseous and dusty debris, and discuss more generally the survivability of dust in elliptical orbits for the larger sample of white dwarfs.

Dhillon, Vik (University of Sheffield)
HiPERCAM - a new tool for the study of eclipsing and pulsating WDs

I will review the design and predicted performance of HiPERCAM, a new five-channel, high-speed imaging photometer that is planned to be mounted on the 4.2m WHT and 10.4m GTC on La Palma from 2017.

Dufour, Patrick (Université de Montréal), Bergeron, Pierre (Université de Montréal), Fontaine, Gilles (Université de Montréal), Blouin, Simon (Université de Montréal)
The montreal white dwarf database: a tool for the community
The last decade or so has seen a dramatic increase in the number of known white dwarf stars, going from about 2500 at the beginning of the millennium to more than 40,000 at the time of this writing. With the huge amount of data and analyses now available, keeping an eye one the big picture has become increasingly difficult. Many basic questions now require a tremendous amount of work just to get updated and accurate answers. Questions such as: How complete is our census of white dwarfs within a given distance of the Sun? How many have metals, are magnetic, are in binary systems, or are He-rich? What fraction of these are there as a function of effective temperature? What are the properties of any given population? What are their mass distributions, luminosity functions, etc.? To answer those questions, one usually needs to compile all the information from the literature, double check it, look out for updates, and assess how recent data/models change the picture. This is a very time-consuming task that has to be repeated again and again as time moves forward. Here we present the “Montreal White Dwarf Database” (MWDD), an accessible database with sortable/filterable tables and interactive plots that will, when fully completed, allow the community to explore the physical properties of all white dwarfs ever analyzed by the Montreal group, as well as display data and analyses from the literature. I will present its current capability and show how it will continuously be updated to instantly reflect improvements made on both the theoretical and observational fronts.

Dunlap, Bart (University of North Carolina at Chapel Hill), Clemens, Chris (University of North Carolina at Chapel Hill), Fuchs, Josh T. (University of North Carolina at Chapel Hill)
A case study in differential seismology

Using published data and new SOAR telescope photometry and spectroscopy, we present preliminary results of an exercise in differential seismology. ZZ Cetis near the blue edge of the instability strip often do not have enough observed modes to determine their underlying stellar structure. However, multiple stars with similar periods can yield more information than one star considered in isolation. Our case study involves four hot ZZ Cetis (L19-2 and friends) with strikingly similar pulsation periods. We explore whether small differences in the observed periods can be identified with structural differences in model stars, which we construct using WDEC and MESA.

Falcon, Ross (Sandia National Laboratories)
Reaching higher densities for our laboratory white dwarf photospheres to test both spectral line shapes and strengths

As part of our laboratory investigation of the theoretical line profiles used in white dwarf atmosphere models, we extend the electron-density range measured by our experiments to higher densities. Whereas inferred parameters using the H-beta spectral line agree among different line-shape models for electron densities less than ~30e16 cm^{-3}, we now see divergence between models. These are densities beyond which have been benchmarked in the laboratory, meaning theoretical profiles in this regime have not been fully validated. Exploring higher densities offers us more leverage while investigating the relative line shapes of hydrogen Balmer lines. It also aides in our study of relative line strengths as we hone our measurement of occupation probabilities.

Fanale, Stephen (University of North Carolina)
Exploring mode variability in the coolest pulsating white dwarfs
We explore the mode stability of long-period pulsations in some of the coolest pulsating white dwarfs with extensive monitoring by the Kepler space telescope, with the aim of observationally constraining growth rates for the oscillation modes.

Farihi, Jay (University College London), Dufour, Patrick (Université de Montréal)
Constraining the nature of DQ stars via pollution frequency
The frequency of atmospheric metals in typical cool white dwarfs has been well established; around 30% of these otherwise-pure, H and He atmosphere stars are externally polluted with heavy elements. In apparent contrast, within the relatively common subset of He-rich DQ stars, there is only a single example known to have further atmospheric metals (Procyon B) among the 224 white dwarfs known within 25pc of the Sun(!). In order to understand this apparent discrepancy, we have begun a program to detect Ca II absorption in DQ stars with UVES on the VLT. We will present the first results from this program, whose aim is to measure robustly, for the first time, the actual frequency and abundances of atmospheric metals in the DQs. Our ongoing study has implications for the origin of the DQ stars and can potentially provide a model-independent, relative measure of stellar properties such as convection zone sizes and evolutionary state.
Feng, Wanda (Arizona State University), Desch, Steven (Arizona State University)
Disk accretion of tidally disrupted rocky bodies onto white dwarfs
About 1/3 of cool white dwarfs are polluted with heavy elements that should sediment out of their atmospheres on astronomically short timescales unless replenished by accretion from a reservoir. Direct accretion of planetesimals is too improbable, and Poynting-Robertson drag of dust is too slow due to low stellar luminosities. The leading model is that white dwarfs accrete from a disk of gas and solid particles, fed by tidal disruption of planetessmals inside the stellar Roche limit. A few disks with detectable gas components have been directly observed, through emission from Ca II atoms in Keplerian rotation. Models can successfully explain the accretion rates of metals onto the white dwarf, provided the gaseous disk viscously spreads at rates consistent with a partially suppressed magnetorotational instability. However, these models currently do not explore the likely extent of the magnetorotational instability in disks by calculating the degree of ionization, or suppression by strong magnetic field. We present a 1-D model of a gaseous white dwarf disk accretion, to assess the extent of the magnetorotational instability in these disks. We infer disk composition from element relative abundances inferred from published spectroscopic studies. By balancing photoionization and thermionic ionization with recombination mechanisms, we can estimate the ionization fraction of disk material. This allows us to estimate the accretion disk viscosity parameter alpha. Magnetic field strengths consistent with white dwarf dipolar magnetic fields are assumed. Elsasser numbers are calculated as a function of radius in the disk. We calculate alpha as a function of disk radius, which is a necessary input to disk models such as those by Metzger et al. (2012).
Fontaine, Gilles (Université de Montréal), Brassard, P. (Université de Montréal), Dufour, P. (Université de Montréal), Xu, S. (European Southern Observatory)
Another look at the C-pollution problem in He-atmosphere white dwarfs
In the light of recent determinations of the abundance of carbon in the atmospheres of known DB and DBZ stars, we reexamine the question of the origin of that element in these objects. While there seems to be little doubt that the carbon seen in the DQ stars is primordial, there are hints that it can be either primordial or accreted (presumably from debris disks) in the DB stars of interest. More generally, there appears to be a dichotomy in the behavior of the C-abundance patterns between the DB stars and the cooler DQ white dwarfs. We review current ideas to account for these puzzling results.
Fuchs, Joshua (University of North Carolina at Chapel Hill), Clemens, Chris (University of North Carolina at Chapel Hill), Dunlap, Bart H. (University of North Carolina at Chapel Hill), Meza, Jesus A. (University of North Carolina at Chapel Hill), et al.
Understanding systematics in ZZ Ceti model fitting to enable differential seismology
We are conducting a survey of all ZZ Cetis south of +10 declination using the SOAR Goodman Spectrograph. Because it employs a single instrument with high UV throughput, this survey will both improve the S/N of the sample of SDSS ZZ Cetis and provide a uniform dataset for model comparison. We are paying special attention to systematics in the spectral fitting in hopes of producing the best relative effective temperature and surface gravity fits ever achieved. These results will feed constraints into new differential seismology models our group is calculating. We will present initial results for four of the brightest hot ZZ Cetis.
Gänsicke, Boris (University of Warwick), Aungwerojwit, A. (Naresuan University), Marsh, T. R. (University of Warwick), Dhillon, V. S. (University of Sheffield), Sahman, D. I. (University of Sheffield), Veras, D. (University of Warwick), Farihi, J. (UCL), Chote, P. (University of Warwick), Ashley, R. (University of Warwick), Arjyotha, S. (Chiang Rai Rajabhat University), Rattanasoon, S. (National Astronomical Research Institute of Thailand), Littlefair, S. P. (University of Sheffield), Pollacco, D. (University of Warwick), Burleigh, M. R. (University of Leicester)
High-speed photometry of the disintegrating planetesimals around WD 1145+017: evidence for rapid dynamical evolution
We obtained high-speed photometry of the disintegrating planetesimals orbiting the white dwarf WD 1145+017, spanning a period of four weeks. The light curves show a dramatic evolution of the system since the first observations obtained about seven months ago. Multiple transit events are detected in every light curve, which have varying durations (~3–12 minutes) and depths (~10%–60%). The time-averaged extinction is ~11%, much higher than at the time of the Kepler observations. The shortest-duration transits require that the occulting cloud of debris has a few times the size of the white dwarf, longer events are often resolved into the superposition of several individual transits. The transits evolve on timescales of days, both in shape and in depth, with most of them gradually appearing and disappearing over the course of the observing campaign. Several transits can be tracked across multiple nights, all of them recur on periods of ~4.49 hr, indicating multiple planetary debris fragments on nearly identical orbits. Identifying the specific origin of these bodies within this planetary system, and the evolution leading to their current orbits remains a challenging problem.
Garcia-Berro, Enrique (Universitat Politecnica de Catalunya), Sendra, L. (Universitat Politecnica de Catalunya), Torres, S. (Universitat Politecnica de Catalunya), Althaus, L. G. (University of La Plata)
The double cooling sequence of the globular cluster Omega Centauri
Omega Centauri is a massive, old, globular cluster that has been extensively studied over the years because of its peculiar color-magnitude diagram. Specifically, the color-magnitude diagram of this cluster has evident signs of the existence of multiple populations, which are clearly seen in the morphologies of both the main sequence and of the degenerate sequence. Considerable theoretical efforts have been done to model the main sequence, which have led to discover several stellar populations. However, the degenerate sequence has not been hitherto modeled with sufficient detail. Here we present a population synthesis study of the cooling sequence, and compare the properties of the modeled stellar populations with those derived from the analysis of the main-sequence number counts.
Geier, Stephan (University of Tübingen), Kupfer, T. (California Institute of Technology), Heber, U. (Friedrich-Alexander Universität Erlangen-Nürnberg), Ziegerer, E. (Friedrich-Alexander Universität Erlangen-Nürnberg), Irrgang, A. (Friedrich-Alexander Universität Erlangen-Nürnberg) Nemeth, P. (Friedrich-Alexander Universität Erlangen-Nürnberg), Schindewolf, M. (Friedrich-Alexander Universität Erlangen-Nürnberg), Barlow, B. N. (High Point University), Bloemen, S. (Radboud University Nijmegen) et al.
Close binary progenitors and ejected companions of thermonuclear supernovae
Close binaries consisting of compact helium stars (hot subdwarfs) and massive white dwarfs are considered as possible progenitors of thermonuclear supernovae via the so-called double-detonation channel. I will report about an ongoing project to search for such close binary systems and the He-star companions ejected by the supernovae.
Genest-Beaulieu, Cynthia (Université de Montréal), Bergeron, Pierre (Université de Montréal)
A study of the Lyman-alpha line profile in DBA white dwarfs

The hydrogen abundances in DBA white dwarfs determined from optical or UV spectra have been reported to differ significantly in some studies. We revisit this problem using our own model atmospheres and synthetic spectra, and present a theoretical investigation of the Lyman-alpha line profile as a function of effective temperature and hydrogen abundance. We identify one possible solution to this discrepancy and show considerable improvement from a detailed analysis of optical and UV spectra of DBA stars.

Gentile Fusillo, Nicola Pietro (University of Warwick), Raddi, Roberto (University of Warwick), Gänsicke, Boris (University of Warwick), Hermes, J.J. (University of North Carolina), Pala, Anna (University of Warwick)
A catalogue of white dwarf candidates in VST ATLAS DR2
Despite the fact that some of the largest and most advanced observatories in the world are located in the southern hemisphere, the southern sky is still today a largely unexplored territory. This knowledge gap between North and South is very marked for white dwarfs, with only ~15% of the known white dwarfs being south of the equator. VST ATLAS is one of the first surveys to provide deep optical multi-band photometric coverage over a large area of the southern skies. By applying the selection method described in Gentile Fusillo et al. (2015) to the second data release of VST ATLAS combined with proper motions from the APOP survey, we created a catalogue of ~2500 high-confidence, moderately bright (g<=19) southern white dwarf candidates . To date we have confirmed 50 white dwarfs via low- to intermediate-resolution spectroscopy, but the vast majority of our candidates have not yet received any spectroscopic follow up.

Gentile Fusillo, Nicola Pietro (University of Warwick), Gänsicke, Boris T. (University of Warwick), Farihi, Jay (University College London), Koester, Detlev (University of Kiel), Schreiber, Matthias (Universidad de Valparaiso), Pala, Anna (University of Warwick)
Trace hydrogen in DB white dwarfs as a possible signature of water accretion
A small number of white dwarfs with helium-dominated atmospheres contain exceptionally large amounts of hydrogen (H) in their convection zones, with the metal-polluted white dwarf GD 16 being the archetypal example. We report the discovery of a new very similar object: the white dwarf ironically named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated accretion rates and abundances of the metals detected in their photospheres (Mg, Ca, Ti, Fe and Ni) as well as the atmospheric H mass. We find that GD 16 and GD 17 contain 2.2 x 10^24 g and 2.9 x 10^24 g of H respectively. The metal abundance ratios indicate that the metal pollutants recently accreted into the convection zones of the two stars are from predominantly dry debris. However, unlike the metal pollutants, H never diffuses out of the convection zone and therefore acts as possible tracer of past accretion of water-rich debris. By extension, water accretion could explain, at least in part, the trace H detected in many other, less extreme, helium atmosphere white dwarfs. We examine 729 SDSS DB white dwarfs from the sample analysed by Koester &Kepler (2015), and find that H contamination is nearly twice more common in metal-polluted DBs than in the metal-free counterparts. This statistically significant correlation corroborates the hypothesis that water-accretion contributes to the trace H in DBA white dwarfs and also suggests that some amount of water must be commonly present in planetary systems with rocky planets.

Gesicki, Krzysztof (Centre for Astronomy, UMK Torun), Miller Bertolami, M. M. (Instituto de Astrofísica de La Plata), Zijlstra, A. A. (Jodrell Bank Centre for Astrophysics)
Post-AGB evolution much faster than previously thought

Two years ago Gesicki et al. (2014) published a detailed analysis of 31 Galactic bulge planetary nebulae (PNe) and based on a simple scaling of the then available models they proposed a faster post-AGB evolution. Coincidentally, during the last White Dwarf Workshop, Miller Bertolami (2015) presented new models for the evolution of central stars of PNe obtaining shorter post-AGB timescales on purely theoretical grounds. In this contribution we present the final masses, initial masses and total ages of central stars of PNe for the same old data sample however interpolated among the new evolutionary sequences. The stellar temperatures cluster around log(Teff) 4.7-4.8 and kinetic ages about 2kyr but the new evolutionary tracks also cluster near this area so the net result is a flat histogram of the derived stellar final masses confined between 0.53 and 0.58 Msun. Consequently, the inferred star formation history is well restricted between 5 and 11 Gyr and is quite compatible with other published studies. The bulge appears to be old and formed over an extended period of time. The new evolutionary tracks proved very good as a tool for analysis of late stages of stars life. The faster than previously thought post-AGB evolution is now well established.

Giammichele, Noemi (IRAP / CNRS - Observatoire Midi-Pyrénées), Charpinet, S. (IRAP / CNRS - Observatoire Midi-Pyrénées), Fontaine, G. (Université de Montréal), Brassard, P. (Université de Montréal)
The core stratification of Kepler white dwarf pulsators: towards observational constraints on nuclear burning and chemical mixing of He-Core-Burning phases.
We present the results of the asteroseismic analyses of a selected sample of white dwarf stars in the Kepler and Kepler2 fields. Our seismic procedure using the forward method based on physically sound, static models, includes a new core parameterization leading us to reproduce the periods of these stars at the precision of the observations. These new fits outperform current state-of-the-art standards by order of magnitudes. We precisely establish the internal structures of these stars and unravel the inner C/O stratification of their core. By studying their internal chemical compositions, and more precisely the C/O profiles, this opens up interesting perspectives on better constraining key processes in stellar physics such as nuclear burning, convection, and mixing, that shape this stratification over time.
Green, Matthew (University of Warwick), Marsh, Tom (University of Warwick), Steeghs, Danny (University of Warwick)
An update on Gaia14aae: The first fully-eclipsing AM CVn

Gaia14aae is a member of the class of cataclysmic variables called AM CVns, which consist of a white dwarf accreting He from its donor. With periods of 5 to 65 minutes, AM CVns include the shortest period binaries containing white dwarfs. AM CVns are believed to form by one of three formation channels, which are most easily distinguished by the nature of the donor star. In the two most probable channels the system undergoes two common envelope phases, after which the donor star is left as either a white dwarf or a non-degenerate He star depending on the channel. AM CVns from the third channel evolve directly from H-accreting cataclysmic variables, and have hot donors with traces of atmospheric H. The relative contributions of these three channels are an area of current interest. Gaia14aae eclipses on a period of 50 minutes, and is the only known AM CVn in which the white dwarf is fully eclipsed. This makes it an attractive system for attempts to constrain the properties of the donor star. We present an update on our attempts to measure these properties, using high-speed multicolour photometry. Preliminary results suggest that the donor star is not degenerate, making the He-star formation channel the most likely; this is consistent with roughly half of the other known AM CVns.

Greiss, Sandra (Lyst)
Deep neural network to classify white dwarfs
In my pervious life, I was an astronomer and one of the big tasks many PhD students face is manual stellar classification. But why ask a student to do what a machine should be able to do too? In this talk, I use the spectra of identified stars (mostly from SDSS) to build a classifier which will detect the identity of the stars. Using a very simple non linear support vector machine, I achieve an 86% accuracy with my model. The next step is to use deep neural networks to achieve a better accuracy. The idea is to build a model that will detect WDs, using stellar spectra. Both machine learning and deep learning methods will be described in this talk.
Van Grootel, Valérie (Université de Liège), Fontaine, G. (Université de Montréal), Brassard, P. (Université de Montréal), Dupret, M.-A. (Université de Liège)
The theoretical instability strip of V777 Her white dwarfs
We present a new theoretical investigation of the instability strip of V777 Her (DBV) white dwarfs. We apply a time-dependent convection (TDC) treatment to cooling models of DB and DBA white dwarfs. Using the spectroscopic calibration for the convective efficiency, ML2/alpha=1.25, we find a wide strip covering the range of effective temperature from 30,000 K down to about 22,000 K at log g = 8.0. This accounts very well for the empirical instability strip derived from a new accurate and homogenous spectroscopic analysis of known pulsators. Our approach leads to an exact description of the blue edge and to a correct understanding of the onset and development of pulsational instabilities, similarly to our results of TDC applied to ZZ Ceti white dwarfs in the recent past. We show that, contrarily to what is generally believed, there is practically no fuzziness on the boundaries of the V777 Her instability strip due to traces of hydrogen in the atmospheres of some of these helium-dominated-atmosphere stars. Contrary to the blue edge, the red edge provided by TDC computations is far too cool compared to the empirical one. A similar situation was observed for the ZZ Ceti stars as well. We hence test the energy leakage argument (i.e., the red edge occurs when the thermal timescale in the driving region becomes equal to the critical period beyond which gravity modes cease to exist), which was successful to correctly reproduce the red edge of ZZ Ceti white dwarfs. Based on this argument, the red edge is qualitatively well reproduced as indicated above. However, upon close inspection, it may be about 1000 K too cool compared to the empirical one, although the latter relies on a few objects only. We also test the hypothesis of including turbulent pressure in our TDC computations in order to provide an alternate physical mechanism to account for the red edge. First promising results are presented.
Guo, Jincheng (National Astronomical Obser. of China), Tziamtzis, Anestis (SHAO), Wang, Zhongxiang (SHAO), Liu, Jifeng (NAOC), Zhao, Jingkun (NAOC), Wang, Song (NAOC)
Serendipitous discovery of a candidate debris disk around the DA white dwarf SDSS J114404.74+052951.6

We report our serendipitous discovery of a candidate debris disk around the recently identified DA white dwarf (WD) SDSS J114404.74+052951.6. The Ca II infrared (IR) triplet, while weakly detected in the source’s optical spectrum, shows a double-peak like profile, and near-IR excesses are also detected from broadband imaging. The two features are commonly seen in the WD gaseous debris disks, and thus indicate the existence of such a disk around the DA WD. We further analyze the emission lines and broadband IR excesses and estimate that the debris disk may have a size of 20–50 RWD and an inclination angle of 82°. The estimated temperature and cooling age for this WD are 23,027 K and ~ 21 Myr, respectively, making it possibly one of the hottest and youngest among candidates of WD debris-disk systems. These properties are also in line with the likely conditions for having a sufficiently bright debris disk. Further observations of the source are warranted in order to determine the debris disk’s properties and search for more absorption features of heavy elements.

Hallakoun, Na'ama (Tel-Aviv University - ESO), Dhillon, V. S. (University of Sheffield), Ivanov, V. D. (ESO), Maoz, D. (Tel-Aviv University), Marsh, T. R. (University of Warwick), Xu, S. (ESO)
WD 1145+017: Multiband fast-photometry of a white dwarf with a disintegrating asteroid

The recent discovery (Vanderburg et al. 2015) of a disintegrating asteroid transiting a white dwarf offers a special opportunity to study the end state of planetary systems. I will present preliminary results of 6 night-long multiband fast-photometry observations of the system, using ULTRACAM and SOFI on the NTT. These might include new constraints on the dust properties, as well as a possible detection of shallower transit periods, previously undetected from ground-based followup observations.

Hambly, Nigel (University of Edinburgh), Rowell, Nick (University of Edinburgh), Lam, Marco (University of Edinburgh)
Get ready for Gaia: cool white dwarfs in common proper motion with Tycho stars

This poster discusses the upcoming (September 2016) Gaia Data Release 1 and preliminary work on maximising the benefit for cool WD science in advance of the full parallax catalogue which will appear around one year later in DR2. The Tycho catalogue is used in conjunction with the all-sky ground based astrometric/photometric SuperCOSMOS Sky Survey in order to identify candidate faint common proper motion objects to the Tycho stars. At the time of writing it is likely that Gaia DR1 will be supplemented by the so-called Tycho-Gaia Astrometric Solution catalogue containing some 2 million parallaxes with Hipparcos-like precision for Tycho stars. While hotter, brighter WDs are present in Tycho, cooler examples are much rarer (if present at all) and CPM offers one method to infer precision distances for a statistically useful sample of these very faint WDs.

Hardy, Adam (Universidad de Valparaiso), Schreiber, Matthias R. (Universidad de Valparaiso), Parsons, Steven G. (Universidad de Valparaiso)
The detection of dust around NN Ser

Eclipse timing variations observed from the post common-envelope binary (PCEB) NN Ser offer strong evidence in favour of circumbinary planets existing around PCEBs. If real, these planets may be accompanied by a disc of dust. I here present the ALMA detection of flux at 1.3 mm from NN Ser, which is likely due to thermal emission from a dust disc of mass ~ 0.8 ± 0.2 Mearth. We performed simulations of the history of NN Ser to determine possible origins of this dust, and conclude that the most likely origin is, in fact, common-envelope material which was not expelled from the system and instead formed a fallback disc. These fallback discs have been predicted by theory but previously remained undetected. While the presence of this dust does not prove the existence of planets around NN Ser, it adds credibility to the possibility of planets forming from fallback material in a ‘second-generation’ scenario.

Hardy, François (Université de Montréal), Dufour, Patrick (Université de Montréal)
Magnetic white dwarfs with heavy elements

Using our newly developed model atmosphere code appropriate for magnetic white dwarfs with metal lines in the Paschen-Back regime, we study various objects of spectral type DQ and DZ. We explore the effects of various parameters such as the field geometry and the convective efficiency and present grids of magnetic synthetic spectra covering the parameter space where these objects are found.

Hayashi, Takayuki (Nagoya University)
White dwarf mass estimation with X-ray spectroscopy
In Intermediate Polars (IPs), plasma flow structure, and therefore, X-ray spectrum emitted by the plasma flow present the WD gravitational potential. Many WD masses in intermediate polars (IPs) were measured with their X-ray spectra. For the X-ray WD mass estimation, multi-temperature X-ray spectral models have been used which was developed by summing up X-ray spectra emitted from the top to the bottom of the plasma flow according to hydrodynamical calculation. By contrast, X-ray reflection from the WD, another main component in the IP's spectrum, was poorly modeled until today. Then I modeled the X-ray reflection with Monte-Carlo simulation. In my simulation, the plasma flow irradiates a cool and spherical WD with the spectra corresponding its position calculated in the thermal spectral modeling. Coherent and incoherent scatterings, photoelectric absorption and K-alpha and K-beta reemissions of Fe and Ni were considered. The constructed spectral model were applied to EX Hya and V1223 Sgr observed by Suzaku and their WD masses were estimated at 0.65+0.11-0.12 Msun and 0.91+0.08-0.03 Msun, respectively. I will introduce the latest X-ray WD mass estimation method.

Heber, Ulrich (Friedrich-Alexander Universität Erlangen-Nürnberg)
Stripped red giants - Helium core WD progenitors and their sdB siblings

Some gaps in the mosaic of binary star evolution have recently been filled by the discoveries of helium-core WD progenitors (often called extremely low mass WDs) as stripped cores of first-giant branch objects. Two varieties can be distinguished. One class is made up by SB1 binaries, companions being WDs as well, another class, the so-called EL CVn stars, are composite spectrum binaries, with A-Type companions. Pulsating stars are found among both classes. A riddle is posed by the apparently single objects. There is a one-to-one correspondence of the phenomena found for these new classes of star to those observed for sdB stars. In fact, standard evolutionary scenarios explain the origin of sdB stars as red giants that have been stripped close to the tip of first red giant branch. I shall review the common properties of both classes of star in comparison and discuss their impact on the formation of low mass white dwarfs.

Hermes, JJ (University of North Carolina at Chapel Hill), Gaensicke, Boris (University of Warwick), Hollands, Mark (University of Warwick)
When flux standards go wild: white dwarfs in the age of Kepler
White dwarfs have been used as flux standards for decades, thanks to their staid simplicity. However, long-term monitoring by the Kepler spacecraft has shown that a handful of white dwarfs that should have purely radiative atmospheres can exhibit significant (several percent) photometric variability, rendering them unsuitable as flux standards. For example, we show that a hot (roughly 100,000 K) apparently single white dwarf exhibits 6% variability, likely from reflection off a close dM companion orbiting every 19.9 hr. We also find that magnetism, even in >50,000 K white dwarfs, can lead to wild flux variations on the timescale of the white dwarf rotation period (days). We emphasize that binarity and magnetism are two extremely important attributes to rule out when establishing white dwarfs as spectrophotometric standards.
Hermes, JJ (University of North Carolina at Chapel Hill), Bell, Keaton (University of Texas at Austin), Montgomery, M. H. (University of Texas at Austin), et al.
Understanding outbursts in the coolest pulsating white dwarfs

The unblinking photometry of the Kepler spacecraft has enabled us to discover a completely unexpected new physical phenomenon: large-amplitude outbursts in the coolest pulsating white dwarfs (see talk by Keaton Bell). I will show that the pulsations respond to the outbursts, and propose that their mode energy is the source of the luminosity increases. I will focus on possible physical mechanisms to explain these rogue waves, which sporadically increase the overall white dwarf flux by up to 15%.

Hermes, JJ (University of North Carolina at Chapel Hill), Gaensicke, Boris (University of Warwick), Breedt, Elme (University of Warwick)
sdA are overwhelmingly not extremely low-mass white dwarf

In a search for new white dwarfs in DR12 of the Sloan Digital Sky Survey, Kepler et al. (2016) found atmospheric parameters for thousands of objects with effective temperatures below 20,000 K and surface gravities between 5.5 < log(g) < 6.5. They classified these objects as cool subdwarfs -- sdA -- and speculated that many may be extremely low-mass (ELM) white dwarfs (helium-core white dwarfs with masses below 0.3 Msun). We present evidence that the vast majority (>99.9%) of these objects are not ELM white dwarfs. Their true identity remains an interesting question.

Hillwig, Todd (Valparaiso University), Jones, David (Instituto de Astrofisica de Canarias), Frew, David (The University of Hong Kong), De Marco, Orsola (Macquarie University)
Binarity in the central stars of planetary nebulae and its relationship to stellar evolution - an observational perspective

The existing status of our knowledge of binary central stars of planetary nebulae will be explored. Binary modeling of known systems is providing physical parameters that can be compared amongst the sample, with nebular characteristics, and with similar binaries with no associated planetary nebula. Correlations among these areas will be discussed, especially in relation to our understanding of stellar evolution.
von Hippel, Ted (ERAU & Max Planck Institute), Webster, Aaron (ERAU), Si, Shijing (Imperial College London), van Dyk, David (Imperial College London), Robinson, Elliot (Argiope), Stenning, David (Sorbonne), Jeffery, Elizabeth (BYU), Stein, Nathan (UPenn), Jefferys, William H. (University of Texas)
The age of the Galaxy: early thoughts based on Gaia white dwarf data

We report on the quality of the first Gaia data release as it pertains to white dwarf distances and proper motions. We apply a self-consistent Bayesian statistical approach to fit modern model ingredients to the available data and derive posterior age distributions for ancient white dwarfs from the Galactic disk, thick disk, and halo. We additionally discuss hierarchical statistical models, which have the potential to better leverage the age information of the rare halo WDs.

Holberg, Jay (Lunar and Planetary Laboratory, University of Arizona), Oswalt, T. D. (Embry-Riddle Aeronautical University), Sion, E. M. (Villanova University)
A tale of two surveys
We compare two white dwarf survey populations. A recent all-sky, distance-limited population of nearby white dwarfs extending to 25 pc that contains 232 members, and a large magnitude-limited spectroscopic population, the SDSS DR7 survey, which contains over 12,000 DA stars. We derive distances and interstellar reddening estimates for the DR7 stars and compute luminosities and ages. Various aspects of the two samples are compared, including mass distributions, luminosity distributions, and cooling age distributions.
Hollands, Mark (University of Warwick), Gaensicke, Boris (University of Warwick), Detlev Koester (University of Kiel)
A large sample of cool, strongly-polluted DZ white dwarfs
White dwarfs with metals in their photospheres are now known to arise from the accretion of extrasolar planetesimals. Study of the coolest of these systems provides both a testbed for model atmospheres under extreme conditions, and allows the chemistry of the accreted parent bodies to be studied directly. We have identified 231 SDSS DZ white dwarfs with extreme line blanketing from metals, and effective temperatures between 4400 and 9000 K. We use our improved model atmosphere code to fit the spectra of this sample, revealing wildly varying compositions, with some systems indicating highly differentiated parent bodies. Analysis of our full sample places constraints on the lifetime of post-main sequence planetary systems, as well as the incidence of magnetism for the oldest degenerate stars.
Hoyer, Denny (University of Tübingen), Rauch, T. (University of Tübingen), Werner, K. (University of Tübingen)
Is the DO-type white dwarf RE0503-289 a unique object?

Trans-iron group elements (atomic numbers Z > 28) are produced by s-process nucleosynthesis on the Asymptotic Giant Branch (AGB). Flash induced mixing during a late thermal pulse (LTP) should strongly enrich their photospheric abundances. While they are so far not found in the hydrogen-deficient post-LTP [WC]- and PG1159-type stars, they were identified in a small number of DO-type white dwarfs (WDs). Abundance determinations have shown that these are indeed up to 1000 times solar. The detection of Ge (Z = 32) in some DA-type WDs yields about solar abundances. Thus, an LTP scenario may be a pre-requisite for very high trans-iron group element abundances. Recently, a very high number of lines of many trans-iron group elements were discovered in the ultraviolet (UV) spectrum of the DO-type WD RE0503-289. Abundance analyses by means of non-local thermodynamic equilibrium (NLTE) model-atmosphere techniques have shown that these elements are strongly overabundant (up to about 4 dex) compared to solar values. To prove the suggestion that an LTP is necessary to yield such overabundances, we performed UV spectroscopy with HST/COS of the PG1159-type star PG1707+427 and of two DO-type stars, namely PG0109+111 and WD0111+002. These three stars are located in the Hertzsprung Russell diagram close before and after the so-called PG1159 wind limit. At about this limit, PG1159-type stars evolve into DO-type WDs. These new observations will show whether the high abundances of trans-iron group elements are a common phenomenon and establish constraints for AGB nucleosynthesis and post-LTP evolution, we perform an advanced NLTE spectral analysis. We present our preliminary results.

Isakova, Polina (Institute of Astronomy of the RAS, Moscow), Zhilkin, A. G. (Institute of Astronomy of the RAS, Moscow), Bisikalo, D. V. (Institute of Astronomy of the RAS, Moscow)
The flow structure in the vicinity of the inner lagrangian point in magnetic cataclysmic variables
The mass transfer between the components of the mCVs occurs through the inner Lagrangian point. The results of 3D numerical simulations of the flow structure in mCVs show that the magnetic field of the accretor essentially influences the flow structure. In the polars (with the magnetic field of 10 – 100 MG at the surface of the white dwarf) the material stream from the donor splits into the several flows as soon as the material leaves the inner Lagrangian point. These flows move along the magnetic field lines and reach the magnetic poles of the accretor. This picture does not correspond to the classical situation of the flow formation when it moves from the donor into the Roche lobe of the accretor along the ballistic trajectory. The aim of our study is to investigate in details the features of the flow splitting in the vicinity of the inner Lagrangian point in classical polars.

Isern, Jordi (ICE-CSIC/IEEC), García-Berro, Enrique (UPC-IEEC), Külebi, Baybars (ICE-CSIC/IEEC), Lorén-Aguilar, Pablo (University of Exeter)
Magnetic fields and crystallizing white dwarfs
The evolution of white dwarfs can be described as a cooling process. When the temperature is low enough, the interior experiences a phase transition and crystallizes. Crystallization introduces two new sources of energy, latent heat and chemical sedimentation, and induces the formation of a convective mantle around the solid core. This structure, which is analogous to that of the Earth, could induce the formation of a magnetic field via dynamo mechanism. In this talk we discuss the viability of such a mechanism and its use as a diagnostic tool of crystallization.
Istrate, Alina (University of Wisconsin Milwaukee), Fontaine, Gilles (Université de Montréal)
Asteroseismology of detailed models of low-mass helium white dwarfs including element diffusion and rotational mixing

We report the first results of a systematic investigation of the pulsation properties of a new generation of low-mass helium white dwarf models computed with MESA in its binary evolution mode. Three families of models are considered: (i) basic structures (with no diffusion nor rotation), (ii) diffusion models (including gravitational settling, chemical and thermal diffusion), and (iii) rotation models (with element diffusion and rotation). We investigate both the adiabatic and nonadiabatic properties of these equilibrium structures. This is motivated by the discoveries, in recent years, of g-mode pulsations in pure-H, extremely low-mass (ELM) white dwarfs residing in the extension of the ZZ Ceti instability strip to low gravity, and, even more recently, of p-mode pulsations in much hotter proto-ELM WDs (see, e.g., Gianninas et al. 2016, arXiV160404621). The latter have mixed H-He envelope compositions with helium acting as the essential "fuel" for exciting pulsation modes via the usual kappa-mechanism.

Istrate, Alina (University of Wisconsin Milwaukee), Marchant, Pablo (Argelander Institute for Astronomy), Tauris, Thomas M. (Argelander Institute for Astronomy and Max Planck Institute for Astronomy ), Langer, Norbert (Argelander Institute for Astronomy), Stancliffe, Richard J. (Argelander Institute for Astronomy), Grassitelli, Luca (Argelander Institute for Astronomy)
New grid of models for ELM WDs including element diffusion and rotational mixing
We present our latest results for the modelling of extremely low-mass (ELM) WDs in which, for the first time, we study the combined effects of rotational mixing and element diffusion. In particular, we investigate how their general properties, such as their evolutionary times, the hydrogen envelope mass and their surface composition are effected compared to the case when just element diffusion is included. The formation of these objects through the LMXB channel in environments with different metallicities is modeled using the state-of-the-art stellar evolution code MESA. Rotational mixing is found to counteract the effect of gravitational settling in the surface of young, bloated ELM proto-WDs suggesting that it is the key component needed to explain the observed metals (especially calcium) in their atmospheres.

Jeffery, Elizabeth (Brigham Young University), Campos, F. (University of Texas at Austin), Romero, A. (UFRGS-Brazil), Kepler, S.O. (UFRGS-Brazil)
White dwarfs in the metal rich open cluster NGC 6253
We have obtained 53 g and 19 i individual 600s images of the super metal rich open cluster NGC 6253 with the Gemini-South telescope. These observations will be used to create deep images of the cluster to observe the cluster white dwarfs for the first time. We will analyze the white dwarf luminosity function to measure the cluster's white dwarf age, search for any anomalous features (as has been seen in the similarly metal rich cluster NGC 6791), and constrain the initial-final mass relation at high metallicities. We present an update on these observations and our program to study the formation of white dwarfs in super high metallicity environments.
Jordan, Stefan (ARI/ZAH Heidelberg)
Gaia data release 1
The first Gaia Data Release is planned for late summer 2016 and will consist of celestial positions and mean magnitudes for more than one billion stars. Additionally, proper motions and parallaxes for about two million stars will be published for which a joint astrometric single-star solution based on Gaia measurements and Tycho 2 positions is possible. It will be discussed what WD research can expect from this and the following data releases.
Joyce, Simon (University of Leicester), Barstow, Martin (University of Leicester), Casewell, Sarah (University of Leicester), Holberg, Jay (University of Arizona), Bond, Howard (Pennsylvania State University)
HST spectra of white dwarfs and the mass-radius relation
The white dwarf mass-radius relation is one of the key building blocks in studies of white dwarfs. While this relationship has a well developed theoretical basis, the observational support for this relation has been limited due to the difficulty in obtaining reliable measurements. High precision measurements are also required to differentiate between mass-radius models based on different assumptions for core composition and surface H layer thickness. We will present HST spectra of several white dwarfs which gives us the best chance yet to test the mass-radius relation through observations of the Balmer lines of white dwarfs in Sirius-like binaries. These observations can be used to measure the white dwarf mass using both the spectroscopic method and the gravitational red-shift method. One of the major challenges for this type of observation is scattered light from the main sequence star contaminating the WD spectrum. We will discuss methods that have been developed to correct for scattered light in the spectrum of HR1358b and also other systematic effects which are important for these observations.
Kepler, Souza Oliveira (UFRGS-Brazil), Koester, Detlev (Universität Kiel), Romero, Alejandra Daniela (UFRGS)
Mass distribution for DAs and DBs
We present the mass distribution for all S/N > 15 pure DA white dwarfs detected in the Sloan Digital Sky Survey up to Data Release 12 in 2015, fitted with Koester models for ML2/alpha = 0.8, for Teff > 13000 K, and for DBs with S/N>10, fitted with ML2/alpha = 1.25, for Teff>16000 K. We also present the mass distributions corrected by volume with the 1/Vmax approach, for stars brighter than g=19. Both distributions have a maximum at 0.624 Msun but very distinct shapes.
Kim, Agnes (Penn State Worthington Scranton)
Semi-chaotic behaviors observed in the asteroseismic fitting of GD358
The analysis of 33 years worth of data on GD358 allowed us to assemble a spectrum of 15 l=1 modes, 13 of which constitute a consecutive sequence. This is the longest consecutive sequence of modes we fit formally so far, with high resolution grids. There were some hints before that some periods may be highly sensitive to some parameters in certain areas of parameter space. With the long consecutive sequence of same ell modes in GD358’s pulsation spectrum, we can study the effect such behaviors can have on asteroseismic fitting. We show some examples of large changes in the calculated period spectra due to small changes in a given parameter and discuss the implications for pipeline fitting of white dwarfs.
Korol, Valeriya (Leiden Observatory), Rossi, E. M. (Leiden Observatory), Groot, P. (Radbould University)
Detection of double degenerate white dwarf binaries as EM and GW sources

On the basis of our theoretical understanding of stellar and binary evolution around 10^8 double degenerate WD binaries (DDWDs) are expected to be present in our Galaxy. According to binary population synthesis models, half of these systems should have periods shorter than few hours, so that gravitational wave emission will bring them into contact within a Hubble time, making them dominant low-frequency gravitational wave emitters and guaranteed sources for the eLISA mission. In my talk I will present how many of compact DDWDs we will be able to detect 1) through EM radiation with Gaia and LSST and 2) through GW radiation with eLISA. Additionally, detached DDWDs with orbital periods of 20-40 min represent ideal systems to study the reaction of their internal structure to tidal forces. I will show my preliminary results on the role of tidal interaction in the case of J0651 (detached system with orbital period P=12.75 min).
Kowalski, Piotr (Forschungszentrum Jülich), Blouin, Simon (Université de Montréal)
Infrared opacities in dense atmospheres of cool white dwarf stars
Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the modeling. This is because these atmospheres have extreme densities of up to a few g/cc and thus represent a dense fluid in which strong multi-atomic interactions determine their physics and chemistry. Therefore, ideal-gas-based description of absorption is no longer adequate, which makes the opacities of these atmospheres difficult to model. This is illustrated with severe problems in fitting the near-IR spectra of cool, He-rich stars or by the distortion of the molecular carbon bands in DQp stars. A good description of the IR opacity is essential for proper assignment of the atmospheric parameters of the cool, He-rich stars, including the H content. Using state-of-the-art methods of computational quantum chemistry we have simulated IR absorption of dense He/H media and found that there is a significant IR absorption due to triple collisions between He atoms (He-He-He CIA opacity) and that the H2-He and H-He collision-induced absorptions (CIA) are strongly affected by multi-particle collisions in dense media. We will discuss the results of these simulations together with the spectra of cool stars. We will show that modern atomistic modeling methods could be successfully applied to modeling condensed stellar atmospheres and that, on the other hand, the atmospheres of white dwarfs represent excellent laboratories for testing the physics and chemistry of matter at extreme conditions.
Krzesinski, Jurek (Pedagogical University of Cracow), Stachowski, Greg (Pedagogical University of Cracow)
Current status of the hot WD luminosity function and non-DA to DA WD ratio from SDSS data

Recent advances in the determination of the hot WD luminosity function have reached the point where we have a good agreement between the observed and theoretical DA and non-DA LFs for WDs from SDSS DR4. It was shown (Garcia et al. 2014, Krzesinski et al. 2015) that the main progress in calculation of the DA LF was obtained when the WD sample was restricted to only carbon-oxygen core WDs. However, one remaining effect which could influence the LF and the non-DA to DA ratio is the difference in visibility of helium and hydrogen atmosphere WDs in a magnitude-limited sample. In this work we explore this effect for the SDSS g filter as a function of effective temperature, as well as make an attempt to evaluate data relevant to the WD sample and LFs from more recent data releases of the SDSS.

Lam, Marco (Institute for Astronomy, University of Edinburgh)
Untangling the white dwarf luminosity functions of the thin disc, thick disc and halo
The white dwarf luminosity function of the solar neighbourhood has been studied for decades. However, due to the lack of chemical signature from the progenitor, objects from the 3 major components of the Milky Way in the solar neighbourhood have been studied all together assuming thin disc completely dominates the sample; or separating the thin and thick disc sample from the halo objects by applying large tangential velocity limits. In this work, I will demonstrate the potential of separating the three components probabilistically to study the individual luminosity functions. The untangling of the three populations is essential to the study of star formation history through the inversion of each luminosity function.

Lam, Marco (Institute for Astronomy, University of Edinburgh), Hambly, N. C. (Institute for Astronomy, University of Edinburgh)
White dwarf luminosity functions from the Pan-STARRS 1 3-pi Steradian Survey

Pan-STARRS 1 builders The rapid image response system of Pan-STARRS 1 scans the sky 4 times a year in grizy filters which allows the delivery of high quality astrometric solutions with typically 3 to 4 years of maximum epoch difference. This allows the search of white dwarfs by means of reduced proper motion down to Hr~23. The survey volume is maximised by pixelising the sky to take into account of degree-scale inhomogeneity of the survey. Below 12000K, the photometric parallax was solved for distance, temperature and H/He ratio (provided by the Montreal group). The luminosity functions are then constructed with the maximum volume density estimator that is generalised for a proper motion limited sample. The effect of interstellar-reddening on the WDLF is studied with the state-of-the-art 3D dust map from Pan-STARRS 1.

Latour, Marilyn (Friedrich-Alexander-Universität Erlangen-Nürnberg), Chayer, Pierre (STScI), Green, Elizabeth (University of Arizona)
Spectroscopic analysis of the standard star Feige 34

Feige 34 is a hot sdO star often used as a flux standard. Despite its brightness, little is known about the star. Its effective temperature is above 60,000 K, its surface gravity around log g ~ 6.0 dex, and it has a subsolar helium abundance. Photometric data indicate the presence of a cooler companion, which is however not seen in the optical spectra. I will present the result of our ongoing work on the spectral analysis of the star.

Li, Chun (Beijing Normal University), Fu, Jianning (Beijing Normal University) Fox-Machado, Lester (Universided Nacional Autónoma de México), Su, Jie (Beijing Normal University)
Asteroseismology of ZZ Ceti star WD 0246+326
The internal structures of pulsating white dwarfs can be explored only with asteroseismology. Time series photometric observations were made for the pulsating DA white dwarf (ZZ Ceti star) WD 0246+326 in 2014 and 2015 with either bi-site observation campaign or single site observation. A few frequencies were detected including several multiplets. With the complementarity of earlier observed frequencies present in the literature, the frequencies are identified as either l = 1 or l = 2 modes. From the multiplets, the rotation period of WD 0246+326 is derived. The value of the average period spacing of the l=1 modes indicates that WD 0246+326 may be a massive ZZ Ceti star. Theoretical models were constructed to constrain the stellar mass and the effective temperature by fitting the frequencies of the eigen modes of the models with the observed frequencies.
Lin, Guifang (Yunnan Observatories, Chinese Academy of Sciences), Yan, Li (Yunnan Observatories, Chinese Academy of Sciences)
A numerical experiment in asteroseismology of white dwarfs

All of the asteroseismological studies are based on the models of white dwarfs. In the model of white dwarfs, in order to find the best-fitting model to constrain its structure, gridding the four parameter space is a time-consuming job, and require a large amount of computation. In order to let the searching more efficient, we carry out a numerical experiment using the WDEC code. We pick out a group of frequencies and periods (with l=1 and l=2) at random which is computed by the models with parameters not on the grids of the models. Then we use the periods to constrain the parameters of the white dwarf conversely. We use the WDEC code to recomputed all the models, and shorten the ranges of parameters step by step. The experimental results show that the asteroseismology can constrain the hydrogen mass fraction and helium mass fraction easily. For the parameters of mass and effective temperature, we can give a little range which is very close to the real parameters.

Löbling, Lisa (University of Tübingen)
The Tübingen Model Atom Database: A revised aluminum model atom and its application for the spectral analysis of white dwarfs

Aluminum (Al) nucleosynthesis takes place during the asymptotic giant branch (AGB) phase of stellar evolution. Al abundance determinations in hot white dwarf stars provide constraints to understand this process. Precise abundance measurements require advanced non-local thermodynamic stellar-atmosphere models and reliable atomic data. In the framework of the German Astrophysical Virtual Observatory (GAVO), the Tübingen Model-Atom Database (TMAD, contains ready-to-use model atoms for elements from hydrogen to barium. A revised, elaborated Al model atom has recently been added. We present preliminary stellar-atmosphere models and emergent Al line spectra for the hot white dwarfs G191-B2B and RE 0503-289.

Manseau, Patrick (Université de Montréal), Bergeron, Pierre (Université de Montréal), Green, Elizabeth M. (Steward Observatory, University of Arizona)
Spectroscopic analysis of hybrid white dwarf spectra from the Sloan Digital Sky Survey

We present a model atmosphere analysis of hot (Teff > 30,000 K) white dwarf spectra from the Sloan Digital Sky Survey showing both hydrogen and helium lines, under the assumption of chemically homogeneous and stratified atmospheric compositions. We identify several hybrid white dwarfs in the SDSS that are better explained in terms of chemically stratified atmospheres, where a thin hydrogen atmosphere floats in diffusive equilibrium on top of a more massive helium envelope. We also present an updated analysis of PG 1305-017, the only stratified white dwarf identified in the spectroscopic analysis of DAO white dwarfs of Bergeron et al. (1994). We interpret our results in the general context of the spectral evolution of white dwarfs.

Manser, Christopher (University of Warwick), Gaensicke, Boris (University of Warwick), Koester, Detlev (Universitat Kiel), Marsh, Tom (University of Warwick), Veras, Dimitri (University of Warwick)
The search for variability of metal absorption lines with new HST observations of SDSS J1228+1040
White dwarfs polluted with metals from planetary debris have thus far shown no confident variability in the strength of their photospheric absorption lines. For most DAZs, the diffusion timescales of metals in the white dwarf atmosphere are of the order of hours to days, and so the presence of variability in the strength of the absorption lines should be directly related to a change in the instantaneous accretion rate. We will be presenting the first of our new far ultraviolet spectra of SDSS J1228+1040 observed with HST to search for this variability. The long term program will run over the next two years, but we will be comparing our initial findings to the previous observations that were obtained in 2010. We will also be presenting our new UVES VLT spectrum of the Ca II triplet at ~8600 Å, which is emitted by the gaseous component of the debris disc and reveals velocity information about the disc. Using this UVES spectrum and the eighteen spectra we previously obtained for SDSS J1228+1040, we have generated a new image of the disc in velocity space (Doppler tomogram).
Maoz, Dan (Tel-Aviv University), Badenes, Carles (University of Pittsburgh), Ciardullo, Robin (Penn State University)
The progenitors and lifetimes of planetary nebulae

The exact identity of their planetary nebula (PN) progenitors has never been established for a large and homogeneous sample. We investigate the relationship between PNe and their stellar progenitors in the LMC by means of a statistical comparison between a highly complete spectroscopic catalog of PNe and the spatially resolved age distribution of the underlying stellar populations. We find that most PN progenitors in the LMC have main-sequence lifetimes in a narrow range between 5 and 8 Gyr, which corresponds to masses between 1.2 and 1.0 Msun, and produce PNe that are visible for 27 ± 6 kyr. We tentatively detect a second population of PN progenitors, with main-sequence lifetimes between 35 and 800 Myr, masses between 8.2 and 2.1 Msun, and average PN lifetimes of 11-8+6 kyr. These two distinct populations suggest the existence of at least two physically distinct formation channels for PNe.

Marsh, Tom (University of Warwick), et al.
A pulsing white dwarf synchrotron source
We present the discovery of a binary system that shows very strong brightness pulsations on a period of ~2 minutes, characteristic of a spinning, magnetic compact object. The pulsations, which show a modulation amplitude close to 100% at FUV wavelengths, are seen also in the optical, near infrared (Ks) and even at radio wavelengths. The source emits an exceedingly broad-band spectrum from X-rays to radio, characteristic of synchrotron emission. We will show that the compact object is a white dwarf and that much of the emission originates from its M star binary companion. The system appears to be powered by the spin-down luminosity of the white dwarf which exceeds the observed emission by a factor of 15. The star is unlike any known class of star, but may be evolutionarily related to the magnetic accreting stars known as intermediate polars. Just how the white dwarf generates synchrotron emission from its M dwarf companion is a mystery.

Miller Bertolami, Marcelo Miguel (Institute of Astrophysics La Plata - IALP/CONICET-UNLP), Althaus, L. (UNLP/CONICET), Corsico A. (CONICET/UNLP)
On the formation of DA white dwarfs with thin H-layers
Systematic asteroseismological and spectroscopic studies in the last decade support the belief that that white dwarfs in the solar neighbourhood harbor a broad range of hydrogen-layer thickness. The reasons behind this spread of H-layer masses are not yet understood and usually misunderstood. This is particularly true in the case of winds. In this work we present, and review, how different mechanisms can lead to the formation of DA White Dwarfs with a broad range of H-layers.

Miller Bertolami, Marcelo Miguel (Institute of Astrophysics La Plata - IALP/CONICET-UNLP)
Constraining the post-AGB evolution of low mass stars: preliminary results
The post-asymptotic giant branch (AGB) phase is arguably one of the least understood phases of the evolution of low- and intermediate- mass stars. The rapid evolution of post-AGB stars (10^3–10^4 yr) means few of them are found in Galactic globular clusters, which are the traditional testing ground for stellar evolution. For this reason, the nearest elliptical galaxy M32 provides an alternative testing ground for our understanding of post-AGB evolution (Brown et al. 2008). The recent grid of models constructed in Miller Bertolami (2016) was aimed at the range of masses of interest for the formation of PNe and is not dense enough in the low-mass range (<0.55 Msun). In order to be useful for comparisons with old stellar populations like the Galactic Globular Clusters and M32 a much denser grid at low-masses needs to be constructed. We present results from an ongoing project aimed at constraining the post-AGB evolution of low mass stars by a detailed comparison of post-AGB stellar counts in M32 with state-of-the-art stellar evolution models as well as with recent determinations of post-AGB stars in Galactic stellar clusters.

Montgomery, Mike (University of Texas at Austin), Hermes, JJ (University of North Carolina at Chapel Hill), Winget, Don (University of Texas at Austin), Provencal, Judith (University of Delaware), Dunlap, Bart (University of North Carolina at Chapel Hill), Clemens, Chris (University of North Carolina at Chapel Hill), Fanale, Stephen (University of North Carolina at Chapel Hill)
Insights from pulsation modes bounded by the base of the convection zone
Fanale and Hermes have recently found a dramatic correlation between the width of the Fourier peaks of a pulsation mode and the mode period in DAVs observed with the Kepler spacecraft. Modes with periods less than ~800 s have HWHM set mostly by the frequency resolution of the FT, whereas modes with periods greater than ~800 s have HWHM that are much larger, typically several microHz (see poster). We posit that the qualitative difference in behavior is due to the location of the mode's outer turning point relative to the base of the convection zone. Since the depth of the convection zone rises and falls as the star becomes locally hotter and cooler due to pulsations, the reflection of a mode from the base of the convection zone is not completely coherent as a function of time. Short-period modes have an outer turning point set by the Lamb frequency, which is well beneath the base of the convection zone. Long-period modes, on the other hand, have an outer turning point defined by the base of the convection zone. Thus, we expect the Fourier peaks of long-period modes to have additional broadening, while those with short periods should be unaffected. Using stellar models and assumptions about the mixing length of convection, we find that we are able to reproduce this behavior.

Montgomery, Michael (University of Texas), Gomez, Thomas (University of Texas, Sandia National Laboratories)
A new model for hydrogen opacities in dense plasmas at white dwarf photosphere conditions.

Accurately measuring the masses of white dwarf stars is crucial in many astrophysical contexts, e.g., asteroseismology and cosmochronology. These masses are most commonly determined by fitting a model atmosphere to an observed spectrum; this is known as the spectroscopic method. However, for cases in which more than one method may be employed, there are well known discrepancies between masses determined by the spectroscopic method and those determined by, e.g., astrometric, dynamical, and/or gravitational redshift methods. In an effort to resolve these discrepancies, we are developing a new model of hydrogen in a dense plasma that is a significant departure from previous models. We employ two approaches to this problem: 1) a simulation-based approach, and 2) a semi-analytic approach. The simulation approach treats the perturbing influences of the protons and electrons classically and then solves the time-dependent Schroedinger equation numerically to obtain the broadened profiles. The semi-analytic approach treats the perturbing electrons quantum mechanically and then solves the time-independent Schroedinger equation for the hydrogen atom in the presence of several nearby protons. We have adopted this approach because, near the Inglis-Teller limit, where energy levels begin to cross, perturbation theory is not sufficient to accurately capture the changes in atomic structure due to the nearby protons. Preliminary results for the proton-dependent energy-level structure show that discrete states exist beyond the Inglis-Teller limit, but, due to the averaging of the different proton configurations and the electron broadening, the spectrum appears featureless above this threshold. This approach produces different opacities compared to the standard treatment, and should result in systematically different inferred masses for white dwarf stars. Experiments at Sandia National Laboratories are currently underway to validate these new models, and we have begun modifications to incorporate these models into stellar atmosphere codes.
[Thomas Gomez is the first author of this proceedings]

Montgomery, Michele M. (University of Central Florida)
What simulations may tell us about white dwarf evolution in binaries

In this presentation, we review the three evolutionary channels proposed for the AM CVn subclass of Cataclysmic Variables. To better understand which evolutionary channel fits best to a particular non-eclipsing system, mass ratios are sought from analytical models that incorporate observables like orbital and positive superhump periods. Historical attempts involve Stage B periods of the superoutburst. However, more recent attempts suggest Stage A periods are better choices for determining mass ratio and thus potential evolutionary channels. To shed more light on the conundrum of which stage is best, we present 3D SPH numerical simulations of Stage A and Stage B of a superoutburst in AM CVn system SDSS J090221.35+38941.9. We present our findings that include eliminating a potential evolutionary channel. If time permits, we present our findings-to-date for SU UMa subtype of CVs.

Mukadam, Anjum (University of Washington), Szkody, Paula (University of Washington), Gaensicke, Boris T. (University of Warwick)
Contrasting the accreting white dwarf pulsators with the non-interacting ZZ Ceti stars
Since the discovery of pulsations in GW Librae in 1998, the class of accreting white dwarf pulsators has been growing steadily. Using lessons learnt the hard way, we wish to highlight the similarities and differences between the ZZ Ceti stars and the accreting white dwarf pulsators. ZZ Ceti stars pulsate in a narrow instability strip due to H ionization in their pure H envelopes; their pulsation characteristics depend on their temperature and stellar mass. Models of accreting white dwarfs are found to be pulsationally unstable due to H/HeI ionization zones, and even show a second instability strip around 15000K due to HeII ionization for a He abundance in the range 0.38-0.48. Both these strips merge for a He abundance higher than 0.48 to form a broad instability strip. Empirically, accreting white dwarfs are found to occupy a wide instability strip in the range of 10500--16000K. Their pulsation characteristics should depend on their temperature, stellar mass, and their He abundance. Accreting pulsators undergo outbursts, during which the white dwarf is heated to temperatures well beyond the instability strip and is observed to cease pulsations. The white dwarf then cools to quiescence in a few years as its outer layers cool more than a million times faster than the evolutionary rate. This provides us with an exceptional opportunity, theoretically, to track the evolution of pulsations from the blue edge to quiescence in a few years, while ZZ Ceti stars evolve on Myr timescales. Some accreting pulsators have also been observed to cease pulsations without any apparent evidence of an outburst. This is a distinct difference between this class of pulsators and the non-interacting ZZ Ceti stars. Understanding the similarities and differences between the accreting white dwarf pulsators and their non-interacting counterparts will eventually help us deduce how accretion affects pulsations.

Mukhopadhyay, Banibrata (Indian Institute of Science)
Significantly super-Chandrasekhar limiting mass white dwarfs and their consequences

Since 2012, we have initiated developing systematically the simplistic to rigorous models to prove that highly super-Chandrasekhar (as well as highly sub-Chandrasekhar) limiting mass white dwarfs are possible to exist. We show that the mass of highly magnetized or modified Einstein's gravity induced white dwarfs could be significantly super-Chandrasekhar and such white dwarfs could altogether have a different mass-limit. This discovery has several important consequences, including explanation of peculiar, over- (as well as under-) luminous type Ia supernovae (SNeIa), a key to unravel the evolutionary history of the universe, which are believed to be triggered in white dwarfs having mass close to their limit (conventionally the Chandrasekhar-limit). We arrive at this revelation, first by considering simplistic, spherical, Newtonian white dwarfs with constant magnetic fields. Then we relax the Newtonian assumption and consider the varying fields, obtaining similar results. Finally, we consider a full scale general relativistic magnetohydrodynamic description of white dwarfs allowing their self-consistent departure from a sphere to ellipsoid. Subsequently, we also explore the effects of modified Einstein's gravity. Our finding questions the uniqueness of the Chandrasekhar-limit. It further argues for a possible second standard candle, which has many far reaching implications. In addition, these highly magnetized white dwarfs are adequate to explain soft gamma-ray repeaters and anomalous X-ray pulsars without any requirement of invoking extraordinarily strong, unobserved yet, magnetic fields, as needed in the magnetar model. Finally, based on simpler calculations, these white dwarfs are shown to be much less luminous than their standard counter-parts (of low magnetic fields). This discovery altogether initiates a new field of research.

Neustroev, Vitaly (University of Oulu), Tsygankov, Sergey (Tuorla Observatory, University of Turku), Suleimanov, Valery (University of Tübingen)
Correlated X-ray and optical variability in intermediate polars during their outbursts
Cataclysmic Variables (CV) are close interacting binary systems in which a white dwarf (WD) accretes material from its late-type low mass companion. In the absence of a strong magnetic field, the material transferred from the donor star forms an accretion disc around the WD and gradually spirals down onto its surface. If the mass-transfer rate is low, the accretion discs can suffer a thermal instability caused by hydrogen ionisation. This results in outbursts for which CVs are named. Dwarf novae (DN) are an important subset of CVs, which undergo such outbursts. Intermediate polars (IP) are another subset of CVs in which the magnetic field of the WD is strong enough to disrupt the inner accretion disc to force the accreting material to flow along field lines on to one or both magnetic poles. Outbursts can also arise in the truncated discs of IPs, although only a few such cases are known. The difference in the strength of the magnetic field of a WD in DNe and IPs determines the time evolution of the X-ray radiation during their outbursts. During a DN outburst X-rays are usually suppressed, whereas in an IP outburst the X-ray is expected to increase. Here we present a detailed comparison of the evolution of the optical and X-ray fluxes of two intermediate polars CC Scl and FS Aur during their outbursts.

Nitta, Atsuko (Gemini Observatory), Kepler, S.O. (UFRGS-Brazil), Kulebi, Baybard (ICE-CSIC/IEEC), Koester, Detlev (University of Kiel), Kleinman, Scot (Gemini Observatory), Jordan, Stefan (ARI/ZAH Heidelberg), Castanheira, B. (The University of Texas at Austin)
Are all magnetic white dwarf stars massive?
Our goal is to address the mystery of the origin of magnetic fields in white dwarf stars. One telling clue in this mystery is that spectroscopic mass determinations for magnetic white dwarf stars are consistently higher (averaging ~0.93Msun) than for non-magnetic white dwarf stars (approximately 0.6Msun). Through our white dwarf catalog of the Sloan Digital Sky Survey (SDSS), we have a statistically significant estimate of the distribution of over 1000 magnetic white dwarf stars of different field strengths versus mass and temperature. The low S/N SDSS spectra, however, leave some uncertainty in our identifications of magnetic fields and hamper our ability to simultaneously measure both the Zeeman-splitting and the Stark pressure broadening in these stars, meaning we cannot reliably measure both B and log(g). Obtaining S/N>70 spectra at Gemini for a sample of low field massive magnetic white dwarf stars, we will test our low S/N determinations of magnetic fields, search for ways to measure B and log(g) simultaneously, and ultimately determine if these magnetic fields are likely developed through the star's own surface convection zone, or inherited from massive Ap/Bp progenitors.

Otani, Tomomi (Embry-Riddle Aeronautical University), Oswalt, Terry D. (Embry-Riddle Aeronautical University), Dhital, Saurav (Embry-Riddle Aeronautical University), Amaral, Marc (Cape Cod Community College), Jordan, Riley (Whitman College)
A search for companions to the pulsating sdB Star EC20117-4014

EC 20117-4014 is suspected to be a spectroscopic binary system consisting of an sdB star and F5V star. It was monitored using the SARA-CT telescope in Cerro Tololo, Chile over several observing seasons. Periodic O-C variations were detected in the two highest amplitude pulsations in EC20117-4014, permitting detection of the F5V companion, whose period and semimajor axis were previously unknown. We estimated the expected radial velocity semi-amplitude for this companion. In addition, we investigated the ranges and combinations of masses and semimajor axes of a possible third component that are consistent with the observational data. [Presented by Terry Oswalt on the behalf of Tomomi Otani]

Oswalt, Terry (Embry-Riddle Aeronautical University), Holberg, Jay (University of Arizona), Sion, Edward (Villanova University)
The completeness of the nearby white dwarf sample: let us count the ways

We have recently extended our ongoing survey of the local white dwarf population, effectively doubling the sample volume. Based upon the latest distance estimates, the present 20 and 25 pc samples were estimated to be about 86% and 68% complete, respectively. Here we examine how the completeness depends upon other observables such as apparent magnitude, photometric color, spectral type, etc. The results may provide additional clues to why “Sirius-Like systems” are underrepresented in the extended 25 pc sample and how new single members of the local white dwarf sample may be found.

Pagnotta, Ashley (American Museum of Natural History)
Old novae: an update
We will present preliminary results from our ongoing survey of Galactic novae, looking to identify counterparts and long-term trends in the light curves.

Pala, Anna (University of Warwick), Gänsicke, Boris (University of Warwick)
Fundamental properties of accreting WDs
Zorotovic et al. (2011) showed that the average mass of accreting white dwarfs (WDs) in cataclysmic variables (CVs) is substantially higher (~0.83Msun) than that of single WDs (~0.64Msun), suggesting that CV WDs may grow in mass, with important implications for their potential as progenitors for Type Ia Supernovae (SNe Ia). This finding was based on a sample of ~30 systems with accurate mass measurements, most of them determined from the analysis of light curves in eclipsing systems. Given the fundamental importance of the evolution of accreting WDs, it is necessary to enlarge this sample and to diversify the methods used for measuring masses. Here, we present the result from a HST + VLT study of six accreting white dwarfs. From the COS and X-shooter data we can establish the spectral energy distribution, from the UV to the near-infrared, constraining the WD surface gravity, while the phase-resolved X-shooter observations provide a measure of the mass ratio from the reflex motion of both the WD and the donor star. Combining the UV and optical data, we can measure the white dwarf masses to a few percent, increasing the number of accreting white dwarfs with accurate masses, and ultimately providing an answer to the question whether accreting CV white dwarfs grow in mass.

Parsons, Steven (Universidad de Valparaiso), Rebassa-Mansergas, Alberto (Universitat Politecnica de Catalunya), Schreiber, Matthias (Universidad de Valparaíso), Gänsicke, Boris (University of Warwick), Ren, Juanjuan (Peking University)
A sample of FGK stars with white dwarf companions
The number of white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive FGK type companions have remained elusive due to the large difference in optical brightness between the two stars. In this talk I will present our work to uncover this hidden population of systems using data from the Radial Velocity Experiment (RAVE) survey in the southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in the northern hemisphere. In total we have identified 934 FGK stars with substantial UV excesses. We have obtained HST ultraviolet spectroscopy for 9 of these and have confirmed the presence of a white dwarf in all cases. I will discuss the potential of this sample for testing type Ia supernovae formation channels
Pelisoli, Ingrid (Univ. Fed. do Rio Grande do Sul), Kepler, S. O. (Univ. Fed. do Rio Grande do Sul)
What is the nature of sdA Stars found in SDSS spectra?

The Sloan Digital Sky Survey (SDSS) is an ongoing project of sky mapping, having so far images for one third of the sky and spectra for more than 4.5 million objects. With these data, the number of known white dwarf stars increased at least five-fold. However, all white dwarf catalogues based on SDSS's data up to data release 10 excluded objects with log g lower than 6.5, equivalent to a mass of about 0.2 solar masses, based on the fact that the Universe is not old enough for single stars to have evolved to white dwarfs with these low masses. This selection excluded the so-called extremely-low mass white dwarfs (ELMs), which are products of binary evolution and can show therefore masses lower than the 0.2 solar masses limit. In the hopes of finding the ELM population in the SDSS footprint, an effort initiated by the ELM Survey (Brown et al. 2010, Kilic et al. 2011, Brown et al. 2012, Kilic et al. 2012, Brown et al. 2013, Gianninas et al. 2015, Brown et al. 2016), the catalogue built by Kepler et al. (2016) based on SDSS's DR12 extended the lower limit to log g = 5.5. This revealed thousands of objects in the 5.5 < log g < 6.5 range, which are being spectroscopically classified as "sdA" stars, since they have log g too low for main-sequence A stars and temperature too cool for common sdB/O stars, but need their binarity confirmed to be called ELMs. Notwithstanding, the sdA classification says absolutely nothing about the evolutionary origin of such objects. Many of them, if confirmed as plain A stars with an overestimated log g, have distances that would put them in the Galaxy's halo, where one would not expect a large population of young objects. It is necessary to further study the sdA population in order to understand these objects' true origin.

Preval, Simon (University of Strathclyde), Barstow, Martin (University of Leicester)
Understanding the spectrum of the very hot DA white dwarf PG0948+534

The very hot DA white dwarf PG0948+534 is a poorly understood object. Attempts to model photospheric metal absorption features present in the star's far-UV spectrum by Barstow et al. (2003b) and Dickinson et al. (2012a) were not successful, as the predicted absorption features were pressure-broadened far beyond that observed. Using detailed NLTE model atmospheres calculated with TLUSTY 201 (Hubeny 1988), we are able to reconcile theory with the observed spectrum of PG0948+534. We find that the previous issues reported in Barstow et al (2003b) and Dickinson et al. (2012a) arose due to assumptions made in calculating model atmospheres used in their analyses. We present new measurements of metal abundances, effective temperature, and surface gravity for the aforementioned object.

Provencal, Judith (University of Delaware), Kawaler, Steve (Iowa State University), Hermes, J.J. (University of North Carolina at Chapel Hill)
Kepler observations of the pulsating white dwarf EC14012-1446
EC14012-1446 is a well studied hydrogen atmosphere pulsator (DAV). This star has recently been observed by Kepler as part of Campaign 6. The 1 minute cadence data spans 6819349 s (78.9 days). The resulting Fourier Transform is spectacular, with frequencies spanning 200-8000 muHz. We present the current results of our analysis, identifying over 60 independent frequencies and more than 100 combinations. We address questions of period spacing, rotational splitting, and amplitude and frequency modulation.

Raddi, Roberto (University of Warwick), Catalan, Silvia (NUI Galway), Gaensicke, Boris (University of Warwick)
White dwarfs in wide binaries and their progenitors

Widely separated (~1000 AU) double-degenerate binaries are important laboratories to study the evolution of stars sharing common birth and composition. Recent studies identified 60 such systems and used them for an accurate characterisation of the initial-to-final mass relation that is comparable to open cluster studies. Here, we confirm via low-resolution spectroscopy 14 systems, five of which contain a magnetic white dwarf (DAH). We determine atmospheric parameters of the DA white dwarfs, placing constraints on their progenitor stars, and compare our results to previous studies. We also determine masses and ages of the DAH white dwarfs, assuming spectroscopic parallaxes derived for their DA companions. We discuss the incidence of magnetic fields in wide binaries (~15 %) with respect to the field population, and we analyse the physical properties of the systems (i.e. separations and mass distribution).

Raddi, Roberto (University of Warwick)
Physical properties of white dwarfs from multi-band photometry
We describe a hierarchical Bayesian model to measure the physical parameters (mass, age, distance, interstellar extinction) of single white dwarfs using only broad-band Galex, APASS, and 2MASS photometry. We test our model on a set of DA white dwarfs with well-assessed atmospheric parameters, determined via optical spectroscopy, and subsequently we apply it to a new proper-motion selected all-sky catalogue of white dwarf candidates. Looking forward to the results of the Gaia mission, we show how the posterior distributions of masses and ages are improved with the inclusion of trigonometric parallaxes.
Rauch, Thomas (University of Tübingen), Gamrath, Sebastien (Université de Mons), Quinet, Pascal (Université de Mons), Hoyer, Denny (University of Tübingen), Werner, Klaus (University of Tübingen), Kruk, Jeffrey Walter (NASA Goddard Space Flight Center, Greenbelt, USA)
Heavy metals resisting gravity in white dwarfs?
Spectral lines of heavy metals, identified in high-resolution ultraviolet spectra of the DO-type white dwarf RE 0503-289, allow precise abundance determinations of these species by means of advanced non-local thermodynamic equilibrium stellar-atmosphere models - provided that reliable atomic data is available. Such analyses of Zn (atomic number Z = 30), Ga (31), Ge (32), As (33), Mo (42), Kr (36), Zr (40), Xe (54), and Ba (56) have recently shown that, without exception, their abundances are unexpectedly strong supersolar (up to about 5 dex). This is much higher than predicted by recent asymptotic giant branch nucleosynthesis calculations. Thus, the interplay of gravitational settling and radiative levitation may play an important role for their photospheric prominence.
Rebassa-Mansergas, Alberto (Universitat Politecnica de Catalunya), Anguiano, Borja (Macquarie University), García-Berro, Enrique (Universitat Politecnica de Catalunya), Freeman, Ken (The Australian University), Manser, Chris (University of Warwick), Pala, Anna (University of Warwick), Gänsicke, Boris (University of Warwick), Liu, Xiaowei (Peking University)
Observational constraints on the age-metallicity relation from white dwarf-main sequence binaries

The age-metallicity relation (AMR) is a fundamental observational constrain for understanding how the Galactic disk formed and evolved chemically on time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in getting accurate stellar ages for individual field stars. We have initiated an observational campaign for providing the much needed observational input by using dwarf-main sequence (WDMS) binaries. White dwarfs are natural clocks and can be used to derive accurate ages. Metallicities can be obtained from the main sequence companions. Since the white dwarfs and the main sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this contribution we present the preliminary AMR derived from analysing a pilot sample of 23 WDMS binaries.

Rebassa-Mansergas, Alberto (Universitat Politecnica de Catalunya), Ren, Juanjuan (Peking University), Parsons, Steven (Universidad de Valparaiso), Gänsicke, Boris (University of Warwick), Schreiber, Matthias (Universidad de Valparaiso), Garcia-Berro, Enrique (Universitat Politecnica de Catalunya), Liu, Xiaowei (Peking University), Koester, Detlev (University of Kiel)
The SDSS DR12 catalogue of white dwarf-main sequence binaries
During the last few years we have carried out the effort of mining the SDSS for building the largest and most homogeneous catalogue of white dwarf-main sequence (WDMS) binaries. Such a superb sample has been widely used for constraining several important open problems in modern astrophysics. In this contribution we present the newest version of our catalogue, which now contains 3294 WDMS binaries from SDSS DR12.
Reindl, Nicole (University of Leicester), Rauch, T. (University of Tübingen), Miller Bertolami, M. M. (Institute of Astrophysics La Plata ), Todt, H. (University of Potsdam), Werner, K. (University of Tübingen)
The rapid evolution of the central star of the Stingray Nebula – latest news from the HST

SAO244567 is an unusually fast evolving star. Within twenty years only, it had turned from a B-type star into the central star of the Stingray Nebula. Space- and ground-based observations obtained over the last decades have revealed that its spectrum changes noticeably over just a few years, showing stellar evolution in real time. The low mass of SAO244567 is, however, in strong contradiction to canonical post-asymptotic giant branch evolution. Thus, its fast evolution has been a mystery for decades. We present results of a non-LTE spectral analysis of the recently obtained HST/COS observations, which finally might allow us to shed light on the evolutionary history of this extraordinary object.
Rolland, Benoit (Université de Montréal), Bergeron, Pierre (Université de Montréal), Fontaine, Gilles (Université de Montréal)
On the interpretation of cool, helium-rich white dwarfs showing traces of hydrogen

We present a spectroscopic analysis of cool (Teff < 12,000 K) helium-rich white dwarfs - drawn mostly from the Sloan Digital Sky Survey - also showing spectroscopic traces of hydrogen. We then perform numerical simulations and attempt to interpret these objects (1) in terms of DA white dwarfs whose thin hydrogen atmospheres have been convectively mixed with the deeper and more massive helium envelope, or (2) in terms of DB or DBA stars that have simply cooled off, taking into account the possible accretion of hydrogen from the interstellar medium or other external sources.

Romero, Alejandra (UFRGS-Brazil), Córsico, A. H. (UNLP-CONICET), Castanheira, B. G. (University of Texas), de Gerónimo, F. C. (UNLP-CONICET), Kepler, S. O. (UFRGS), Althaus, L. G. (UNLP-CONICET)
Asteroseismology of Kepler ZZ Ceti stars with fully evolutionary pulsation models

DAV stars, also called ZZ Ceti variables, are pulsating white dwarfs with atmospheres rich in hydrogen. As pulsation is a normal phase of evolution, then asteroseismology of DAV stars can provide valuable clues about the origin, structure and evolution of DA white dwarfs. Recently the Kepler spacecraft observed ~10 ZZ Ceti stars giving the opportunity to study their variability from space observations. We present a study of pulsational properties of the four ZZ Ceti stars observed with the Kepler spacecraft: GD 1212, SDSS J113655.17+040952.6, KIC 11911480 and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf stars, characterized by a detailed and consistent chemical inner profile. Also, we have applied some restrictions by virtue of the detection of rotational frequency splittings in some of the analyzed stars. From our fits we found values of gravity and effective temperature of log g=8.33 and Teff=11127 for GD 1212, log g = 8.20 and Teff = 108750 K for KIC 4552982, log g= 7.91 and Teff= 12640 K for KIC1191480 and log g=7.95 and Teff=12060 K for SDSS J113655.17+040952.6. The asteroseismological values are in good agreement with the spectroscopy, except for GD 1212 which shows an asteroseismological mass higher than the spectroscopic value, but in agreement with photometric determinations. KIC 11911480 and SDSS J113655.17+040952.6 show a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6.

Romero, Alejandra (UFRGS-Brazil), de Gerónimo, Francisco (UNLP-CONICET), Córsico, A. H. (UNLP-CONICET), Althaus, L. G. (UNLP-CONICET)
The impact the C(α, γ)O reaction rate on the asteroseismology of ZZ Ceti stars
As it is known, the 12C(α, γ)16O reaction rate is one of the most important nuclear reaction rates, since it influences the abundance of elements heavier than 12C and the inner chemical stratification of the white dwarf core. Given that the pulsation modes in variable white dwarf stars are sensitive to the inner structure, this reaction rate can be estimated using Asteroseismology. In this work we focus on the uncertainties resulting from the incomplete knowledge of the 12C(α, γ)16O nuclear reaction rate and their impact over the C/O core stratification and pulsation properties. To do this we performed evolutionary computations from ZAMS to the TP-AGB phase and the white dwarf cooling curve, covering an initial mass range of 1-5 Msun with solar metallicity. These evolutionary sequences were computed considering two different reaction rates for 12C(α, γ)16O, both provided by Kunz et al. (2002) as low and high reaction rates. Once in the WD phase, we let the models cool down until they reach the ZZ Ceti instability strip, where we computed adiabatic non-radial g-modes pulsations with periods in between 50 and 1500 s. By comparing with the values obtained with the canonical 12C(α, γ)16O reaction rate from NACRE (Angulo et al. 1999), we estimate the impact of the uncertainties over pulsation periods, period spacing and kinetic energy of the modes. In addition, we performed asteroseismological fits for several ZZ Ceti stars, including the well studied G117-B15A, and compare our results with those from others authors.

Schaeuble, Marc (University of Texas at Austin), Falcon, Ross (Sandia National Laboratories), Gomez, Thomas (University of Texas at Austin), Winget, Don (University of Texas at Austin), Montgomery, Mike (University of Texas at Austin), Bailey, Jim (Sandia National Laboratories)
Helium at white dwarf photosphere conditions: experimental line shapes and shifts
We present preliminary results of an experimental study exploring He line shapes and shifts at photospheric conditions of white dwarf stars. These data were collected as part of the Z Astrophysical Plasma Properties (ZAPP) collaboration on Sandia National Laboratories’ Z-machine, the largest x-ray source on earth. Our helium results have many applications ranging from validating current DB white dwarf atmospheric models to providing accurate He pressure shifts at varying temperatures and densities. In a much broader context, these helium data can also be used to guide theoretical developments in new continuum-lowering models for two-electron atoms. We also discuss future applications of our updated experimental design, which allows us to sample a greater range of densities, temperatures, and gas compositions.

Schreiber, Matthias R. (Universidad de Valparaiso), Zorotovic, Monica (Universidad de Valparaiso), Wijnen, Thomas (University Nijmegen)
The evolution of Cataclysmic Variables as revealed by the masses of their primary stars
Since the first binary population studies for CVs have been performed more than twenty years ago, dramatic disagreement between model predictions and observations have been revealed. The predicted CV space density is orders of magnitudes larger and much less systems are predicted to exist at long orbital periods than is observed. The probably most severe disagreement between theory and observation, however, has been discovered rather recently. The white dwarf masses in Cataclysmic Variables (CVs) have been measured to significantly exceed those of single white dwarfs, which is the opposite of what is theoretically expected. I show that this disagreement cannot be explained as an observational bias nor due to white dwarf mass growth prior or during the CV phase. If, however, an empirical consequential angular momentum loss prescription especially designed to solve the white dwarf mass problem is incorporated in binary population models, the predictions for the white dwarf masses, space density, and orbital period distribution are simultaneously brought into agreement with the observations. I will furthermore show that the large masses of the white dwarfs in CVs allow us to identifiy for the first time strong candidates for detached CVs crossing the period gap. It therefore seems that the white dwarf mass measurements in CVs guided us to a more complete and better understanding of CV evolution. The big remaining question is which physical mechanism is causing the required consequential angular momentum loss. I speculate that frictional angular momentum loss following nova eruptions might cause such an angular momentum loss and could be this missing ingredient of CV evolution.

Schwab, Josiah (UC Berkeley), Lecoanet, Daniel (UC Berkeley), Quataert, Eliot (UC Berkeley), Bildsten, Lars (UCSB/KITP), Timmes, F. X. (ASU), Burns, Keaton J. (MIT), Vasil, Geoffrey M. (U. Sydney), Oishi, Jeffrey S. (Farmingdale State), Brown, Benjamin P. (U. Colorado, Boulder)
Turbulent chemical diffusion in convectively bounded carbon flames

It has been proposed that mixing induced by convective overshoot can disrupt the inward propagation of carbon deflagrations in super-asymptotic giant branch stars. To test this theory, we study an idealized model of convectively bounded carbon flames with 3D hydrodynamic simulations of the Boussinesq equations using the pseudospectral code Dedalus. Because the flame propagation timescale is ~ 10^5 times longer than the convection timescale, we approximate the flame as fixed in space, and only consider its effects on the buoyancy of the fluid. By evolving a passive scalar field, we derive a turbulent chemical diffusivity produced by the convection as a function of height, Dt(z). Convection can stall a flame if the chemical mixing timescale, set by the turbulent chemical diffusivity, Dt, is shorter than the flame propagation timescale, set by the thermal diffusivity, κ, i.e., when Dt > κ. However, we find Dt < κ; for most of the flame because convective plumes are not dense enough to penetrate into the flame. This implies that convective mixing cannot stall a carbon flame and that “hybrid carbon-oxygen-neon” white dwarfs are not a typical product of stellar evolution.

Schwab, Josiah (UC Berkeley), Quataert, Eliot (UC Berkeley), Kasen, Daniel (UC Berkeley)
The evolution and fate of super-Chandrasekhar mass white dwarf merger remnants
We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs. We focus on cases that have a total mass in excess of the Chandrasekhar mass. After the merger, the remnant manifests as an L ~ 3*10^4 Lsun source for ~ 10^4 yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme AGB stars. Roughly ~ 10 such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-center carbon fusion is ignited within the merger remnant and propagates inward via a carbon flame, converting the WD to an oxygen-neon composition. By following the evolution for longer than previous calculations, we demonstrate that after carbon-burning reaches the center, neutrino-cooled Kelvin-Helmholtz contraction leads to off-center neon ignition in remnants with masses > 1.35 Msun. The resulting Ne flame converts the core to a silicon WD. Thus, super-Chandrasekhar WD merger remnants do not undergo electron-capture induced collapse as traditionally assumed. Instead, if the remnant mass remains above the Chandrasekhar mass, we expect that it will form a low-mass iron core and collapse to form a neutron star.
Si, Shijing (Imperial College London)
Bayesian hierarchical models for a group of white dwarfs

In astrophysics, we often aim to estimate a parameter for each of a number of objects in a population. For example, we may want to estimate the age of each member of a sample of halo white dwarf (WD). The standard strategy is to separately study each of the objects using case-by-case analyses, and then in a follow-up analysis, study the distribution of the fitted parameters across the population. In this paper, we develop novel methods that allow us to take advantage of existing software designed for such case-by-case analyses to simultaneously fit parameters of both the individual objects and the parameters that quantify their distribution across the population. Our methods are based on Bayesian hierarchical modelling which is known to produce parameter estimators for the individual objects that are on average closer to their true values than estimators based on case-by-case analyses. We verify this in the context of estimating ages of Halo white dwarfs via a series of simulation studies. Finally, we deploy our new techniques on photometric observations of five candidate Halo WDs to obtain estimates of each of their ages along with an estimate of the mean age of Halo WDs. Although this sample is quite small, this actually lays the ground work for much more substantial studies that will be possible using data collected with the upcoming Gaia mission.

Smith, Allyn (Austin Peay State University), Tucker, Douglas L. (Fermilab), Wester, William (Fermilab), Gulledge, Deborah (APSU), McDonald, Chris (APSU), Robertson, Jacob (APSU), Tremblay, Pier-Emmanuel (University of Warwick)
Update of the large survey calibration white dwarf program

We present an update on the program to develop a sample of well-characterized, white dwarfs suitable for use as calibration sources for large surveys. We will present the latest results of our photometry and spectroscopic observing programs, reductions status, and the synthetic spectroscopy for these objects.

Stone, Nicholas (Columbia University), Metzger, Brian (Columbia University), Loeb, Abraham (Harvard University)
Evaporation and accretion of extrasolar comets following white dwarf kicks
Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass-loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation. The dusty debris from these comets will manifest itself as an IR excess temporarily visible around newborn white dwarfs; examples of such discs may already have been seen in the Helix Nebula, and around several other young white dwarfs. Future observations with the James Webb Space Telescope may distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. Although competing hypotheses exist, the observation that ~15 per cent of young white dwarfs possess such discs, if interpreted as indeed being cometary in origin, provides indirect evidence that low-mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.
Su, Jie (Beijing Normal University), Fu, J. N. (Beijing Normal University), Khokhuntod, P. (Beijing Normal University)
Two new ZZ Ceti stars from the LAMOST survey

We introduce the progress of our work on searching for new pulsating white dwarfs. The candidates were sifted from the published DA white dwarf catalogs based on the data of pilot survey and data release one (DR1) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey. We carried out follow-up photometric observations on tens of candidates. Preliminary, two candidates (LAMOST J004628.31+343319.90 and SDSS J062159.49+252335.9) have been identified as new DA pulsating white dwarfs (ZZ Ceti stars). For the first star LAMOST J004628.31+343319.90, more observations were performed and new photometric data were obtained.

Sullivan, Denis (Victoria University of Wellington)
Time-series spectroscopy and photometry of the hot DBV white dwarf EC20058-5234
We present a new analysis of both time-series spectroscopic and photometric observations (some published and some unpublished) obtained during the interval 2003 to 2013. A Magellan 6.5m telescope was used to obtain the spectroscopic observations and one photometric data set, while the additional photometric data were obtained using the University of Canterbury Mt John Observatory (UCMJO) one metre telescope. Among other things an attempt is made to identify spectral changes resulting from the two dominant pulsation modes.

Toloza Castillo, Odette (University of Warwick), Gänsicke, Boris (University of Warwick)
Constraints on the single degenerate channel from white dwarf atmospheric compositions
Supernova Ia became very popular as they were the key in the discovery of the accelerating expansion of the universe. While it is generally accepted that a Supernova Ia is the product of a white dwarf surpassing the Chandrasekhar limit, the circumstances leading to the ignition are intensely debated. One channel formation is the single degenerate model, involving a white dwarf growing in mass from the ashes of accreting material from a massive companion. Supersoft X-rays sources are strong observational evidence of Supernova Ia progenitors evolving through the single degenerate, however our understanding of these rare objects is very poor. If a supersoft X-ray source fails the ignition the system will morph into a white dwarf accreting from a donor, where the accreting material carries the fingerprint of the CNO process, and these abundances can constrain the supernova Ia progenitors. Here we show preliminary results of chemical composition of 5 systems that we strongly suggest that they evolved through the single degenerate channel.
Torres, Santiago (Universitat Politècnica Catalunya - BarcelonaTech), García-Berro, E. (Universitat Politècnica Catalunya), Cojocaru, R. (Universitat Politècnica Catalunya), Calamida, A. (NOAO - AURA)
A population synthesis study of the white dwarf cooling sequence of the Galactic bulge

Recent observations from the Hubble Space Telescope have determined the white-dwarf cooling sequence of the Galactic bulge for the first time. However, observations show systematically redder objects than theoretical cooling tracks for CO-core predictions. Here we present a population synthesis study of the white dwarf cooling sequence of the galactic bulge including both, single and binary systems. Based on the most up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, for carbon-oxygen and helium cores, and incorporating also detailed prescriptions of the evolution of binary systems and of the observational biases. This has allowed us to model with excellent accuracy the white dwarf population of the Galactic bulge. Among other interesting results we have estimated the fraction of binaries and double degenerate systems of the Galactic bulge.

Torres, Santiago (Universitat Politècnica Catalunya - BarcelonaTech), García-Berro, E. (Universitat Politècnica Catalunya)
The white dwarf luminosity function of stars within 40 pc of the Sun
We present a population synthesis study of the white dwarf population within 40 pc from the Sun, and compare the results of this study with the properties of the observed sample. We use a state-of-the-art population synthesis code based on Monte Carlo techniques that incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases. We find a good agreement between our theoretical models and the observed data. Our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we propose is due to a recent episode of star formation. We also derive the age of the Solar neighborhood employing the position of the observed cut-off of the white dwarf luminosity function, obtaining 8.9 ± 0.2 Gyr. However, the region around the maximum of the white dwarf luminosity function remains controversial, and we argue that the precise shape of that maximum is best explained assuming that the initial-to-final mass relationship is steeper for progenitor masses larger than about 4 solar masses.
Tovmassian, Gagik (IA UNAM)
Missing magnetic WDs in detached close binaries
A substantial fraction of isolated white dwarfs are strongly magnetic. Even a higher number of magnetic white dwarfs are reported in cataclysmic variables (CVs). However, we have been struggling to find them in detached, close binaries. Recently, we identified a pair of such systems with K star companions. We suggest that we could not recognize them, since the coupled magnetic fields of the white dwarf and chromospherically active K star fuel accretion onto the white dwarf replicating a CV behaviour.
Tremblay, Pier-Emmanuel (University of Warwick), Cummings, Jeff (JHU), Kalirai, Jason (STScI)
The field white dwarf mass distribution
We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the volume-complete survey within 20 pc and the SDSS magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disk vertical scale height as a function of stellar age, and binary evolution. Nevertheless, we find that fixed standard assumptions for the above parameters result in predicted mean masses that are in good qualitative agreement with the observed values. It suggests that derived masses for both studied samples are consistent with our current knowledge of stellar and Galactic evolution. Our simulations overpredict by 40-50% the number of massive white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a Salpeter IMF when we account for all biases.. Furthermore, we find no evidence of a population of double white dwarf mergers in the observed mass distributions.
Vauclair, Gerard (IRAP, OMP), Wachlin, F.C. (Instituto de Astrofisica de La Plata, Argentina) Vauclair, S. (IRAP, OMP, Toulouse, France), Althaus, L.G. (Instituto de Astrofisica de La Plata, Argentina)
The role of fingering convection in accreting hydrogen-rich white dwarfs: the case of GD 133 and G 29-38

The accretion of heavy material from debris disks on the surface of hydrogen-rich white dwarfs induces a double diffusivity instability known as the fingering convection. It leads to an efficient extra mixing which brings the accreted material deeper in the star than by considering only mixing in the surface dynamical convection zone, in a time scale much shorter than that of gravitational settling. We performed numerical simulations of a continuous accretion of heavy material having a bulk Earth composition on the two well studied DAZ and ZZ Ceti pulsators GD 133 and G 29-38. We find that the existence of fingering convection implies much larger accretion rates to explain the observed abundances than previous estimates based on the standard mixing length theory and gravitational settling only.

Vauclair, Gerard (IRAP, OMP), Fu, J.N. (Beijing Normal University), Su, J. (Beijing Normal University)
Asteroseismology of the ZZ Ceti and DAZ white dwarf GD 133

GD 133 is a DAZ white dwarf with an atmosphere polluted by heavy elements accreted from a debris disk, which is formed by the disruption of rocky planetesimals with orbits bringing them at the white dwarf tidal radius. To reach such orbits implies the potential presence of a perturbing planet. GD 133 is a ZZ Ceti pulsator close to the blue edge of the instability strip. The presence of a planet could be revealed by the periodical variation of the observed pulsation periods induced by the orbital motion of the white dwarf around the barycenter of the system. We started a multi-site photometric follow-up aimed at detecting the signature of this potential planet. As a preliminary result of this work in progress, we give the parameters of the GD 133 best-fit model derived from asteroseismology.

Vanderbosch, Zachary (University of Texas at Austin), Winget, Don (University of Texas at Austin), Clemens, J. C. (University of North Carolina at Chapel Hill)
V471 Tauri: examining ETVs with two independent clocks

We will present an analysis of binary star V471 Tauri comparing eclipse timing variations (ETVs) with O-C variations seen in the spin period of the white dwarf. ETVs are often considered indirect evidence of a third body interacting via the light-time effect but can result from other mechanisms as well. Along with the orbital period of 0.521-days, the DAZ white dwarf is magnetically active, rotating with a stable spin period of 9.5-min and accreting wind material from its K2 V companion onto optically bright magnetic poles. The spin and orbital periods provide two independent clocks which allow us to test the third-body theory as the light-time effect would influence both clocks equally. Using available data from the past few decades, we will present O-C diagrams for both orbital and spin periods, noting a relatively flat line for the WD spin-period, and argue for alternative mechanisms causing ETVs, namely the Applegate effect. Additionally, we will present other systems which we find promising candidates for Applegate-induced ETVs.

Veras, Dimitri (University of Warwick), Shannon, A. (University of Cambridge), Gänsicke, Boris T. (University of Warwick)
Hydrogen delivery onto DB white dwarfs from remnant exo-Oort cloud comets
The origin of trace hydrogen in white dwarfs (WDs) with He-dominated atmospheres is a long-standing problem, one that cannot satisfactorily be explained by the historically favoured hypothesis of accretion from the interstellar medium. Here we explore the possibility that the gradual accretion of exo-Oort cloud comets, which are a rich source of H, contributes to the apparent increase of trace H with WD cooling age. We determine how often remnant exo-Oort clouds, freshly excited from post-main-sequence stellar mass loss, dynamically inject comets inside the WD's Roche radius. We improve upon previous studies by considering a representative range of single WD masses (0.52-1.00 Solar masses) and incorporating different cloud architectures, giant branch stellar mass loss, stellar flybys, Galactic tides and a realistic escape ellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyr ages of WD cooling. We find that about 10^(-5) of the material in an exo-Oort cloud is typically amassed on to the WD, and that the H deposits accumulate even as the cloud dissipates. This accumulation may account for the relatively large amount of trace H, 10^(22)-10^(25) g, that is determined frequently among WDs with cooling ages ≥ 1 Gyr. Our results also reaffirm the notion that exo-Oort cloud comets are not the primary agents of the metal budgets observed in polluted WD atmospheres.
Voloshina, Irina (Sternberg Astronomical Institute, Moscow State University), Khruzina, Tatiana (Sternberg Astronomical Institute, Moscow State University)
Light curves analysis of deeply eclipsed dwarf nova GY Cnc.
High precision photometry of deeply eclipsed dwarf nova GY Cnc was provided in R band during the period 2013-2016 with two telescopes of Sternberg Astronomical Institute observational station in quiet and active states. A search of periodicities was carried out with the help of Lafler-Kinman method for the all data in quiescence. Obtained value Porb = 0d.1754420 placed GY Cnc above the 2-3 h period gap in the distribution of CVs periods. The shape of the mean phase light curves constructed with this period is variable for quiescence and outburst indicating that the physical properties of the accretion disk are quite different. During our monitoring two outbursts of this dwarf nova were detected: in April 2014 and October 2015. Our data evidence that duration of an outburst for GY Cnc is approximately 10 days. During such outbursts the brightness increased on ~ 3.5 mag. The Rc light curve obtained in an active state shows a reflection effect and a possible shallow secondary eclipse around phase 0.5. In frames of combined model worked out for eclipsing CV-s by Khruzina (2011), the observational light curves were fitted with theoretical ones, parameters of the system components and their contribution to the total brightness of GY Cnc were estimated. The model gives a good quantitative match to the observations. From analysis of the brightness changes in quiescence we found that the range of the maximal brightness changes is delta R ~ 15m.7–16 m.4 and the depth of minimum (Rmean ~ 17 m.1),- in the range delta R ~ 16m.6–17 m.2.
Wada, Yuuki (University of Tokyo), Yuasa, Takayuki (University of Tokyo), Nakazawa, Kazuhiro (University of Tokyo), Makishima, Kazuo (RIKEN), Hayashi, Takayuki (GSFC/NASA, Nagoya University), Ishida, Manabu (ISAS/JAXA)
NuSTAR observations of the Dwarf Nova GK Persei in 2015: comparison between outburst and quiescent phases
Intermediate Polars are a subset of Cataclysmic Variables, hosting strongly magnetized white dwarfs (WDs). Due to the strong magnetic field, accreting gas is considered to form an accretion disk down to the Alfven radius, then follows the magnetic lines, and finally falls onto the magnetic pole of the WD to form accretion columns. In the accretion column, the gas is heated by a standing shock and cools down by emitting X-rays. The X-ray measured maximum temperature Tmax of the column, which is proportional to the gravitational potential gained by the accreting matter, has been utilized to estimate the WD mass (e.g. Ishida et al. 1991; Fujimoto et al. 1997; Cropper et al. 1998; Suleimanov et al. 2005; Yuasa et al. 2010; Hayashi et al. 2011). GK Persei is an Intermediate Polar and exhibits Dwarf Nova outbursts for ~ 2 months every ~ 2 years. We present results of its observations during an outburst in March 2015 (ToO) and a quiescence in September 2015 (PI: T. Yuasa), both performed by NuSTAR. The 5-50 keV flux during the outburst was 25 times higher than that during the quiescence. With a multi-temperature emission model and a reflection model, we derived Tmax=19.6 (+/- 0.5) keV in the outburst, and 37.1 (+3.5/-3.1) keV in the quiescence. The significant change in Tmax, in response to the flux variation, is utilized to better constrain basic parameters of the WD, including the mass, the radius, the magnetic-field strength, and the shock height.
Werner, Klaus (University of Tübingen), Kruk, J. W. (Goddard Space Flight Center), Rauch, T. (University of Tübingen)
Trace metals in PG1159 stars

We present results of new spectral analyses of FUSE spectra of PG1159 stars, focusing on trace metal abundances. They are compared to predictions from stellar evolution models.

Williams, Kurtis (Texas A&M University-Commerce), Bellini, Andrea (STScI), Canton, Paul (Univ. Oklahoma), Guadalupe, Valeria (TAMU-C), Cantu, Sarah (TAMU-C)
Early results from a proper motion selected sample of open cluster white dwarfs
Open star cluster white dwarfs (WDs), as members of a co-eval, simple stellar population, have proven to be highly useful tools for studying varied aspects of stellar evolution where high age and metallicity precision are required, such as the high-mass end of the initial-final mass relation. Most cluster WD memberships have been determined primarily by the WDs' distance modulus as calculated from atmospheric parameters. This method, however, will result in interesting populations (such as binary systems) being excluded from samples and likely results in some degree of contamination from field white dwarfs. Work with nearby open clusters has shown the importance of proper motion selection. However, many interesting star clusters are sufficiently distant that proper motion measurements do not yet exist for faint WDs, and these WDs are too faint for GAIA measurements. We present early results from proper motion measurements of WDs in the open clusters Messier 67 and Messier 35 and discuss the future directions of our ongoing survey.
Wilson, David (University of Warwick), Gaensicke, Boris (University of Warwick), Holberg, Jay (University of Leicester), Koester, Detlev (Universitat Kiel), Barstow, Martin (University of Leicester), Long, Knox (STScI), Burleigh, Matthew (University of Leicester), Casewell, Sarah (University of Leicester), Preval, Simon (University of Strathclyde), Farihi, Jay (University College London)
FUV observations of the mysterious metal-polluted white dwarf GD 394

GD394 is a hot, extremely metal-polluted white dwarf, which is almost certainly accreting material from a remnant planetary system. EUVE observations in the early 90s revealed a 1.15 day periodicity with a 25 percent amplitude. This was hypothesised to be due to an accretion spot on the white dwarf, rotating into and out of view on the white dwarf spin period. We have obtained phase-resolved HST/STIS high-resolution FUV spectra of GD394 that sample the entire spin period. The spectra contain dozens of strong metal absorption lines, which we use to (1) test the hypothesis of a rotating spot, (2) determine the chemical composition of the planetary system at GD394, and, via comparison with a 1992 GHRS spectrum, (3) explore the variability of metal pollution over a time-scale of decades.
Wolf, Bill (University of California Santa Barbara), Bildsten, Lars (KITP, UCSB)
The fate of persistent supersoft sources
Persistent supersoft sources are strong emitters in the UV and supersoft X-rays that are believed to be the manifestations of white dwarfs undergoing steady and stable surface burning of hydrogen accreted through disk from a companion main sequence or slightly evolved companion. I explore models of the formation of these systems and their long-term evolution through helium flashes, which are not unlike very late thermal pulses from dying AGB stars. I look into the possibility of the only known helium nova, V445 Puppis, being the result of one of these helium flashes rather than the result of direct helium accretion. Finally I'll present a calculation for an estimate of the fraction of observed supersoft sources which could actually be due to the long-term supersoft phase following one of these helium flashes rather than the standard hydrogen burning scenario.
Xu, Siyi (ESO, Garching), Dufour, Patrick (Université de Montréal), Jura, Michael (UCLA), Debes, John (STScI), Su, Kate (University Arizona), Green, Elizabeth (University Arizona), Vanderburg, Andrew (Harvard University), Hallakoun, Na’ama (ESO), Croll, Bryce (Boston University), Ivanov, Valentin (ESO), Rappaport, Saul (MIT), Zuckerman, Ben (UCLA), Muirhead, Philip (Boston University)
Multi-epoch spectroscopic observations of gas around a disintegrating extrasolar asteroid
We report multi-epoch spectroscopic observations of WD 1145+017—a white dwarf that was recently found to have multiple transits from asteroid-sized objects within its tidal radius. In the data obtained in April 2015, we discovered numerous circumstellar absorption lines with linewidths of ~300 km s^-1 from Mg, Ca, Ti, Cr, Mn, Fe, and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe, and Ni and the abundances resemble that of terrestrial material (Xu et al. 2016). Recent observations obtained during 2016 show that the circumstellar lines are rapidly changing. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

Subasavage, John (U. S. Naval Observatory - Flagstaff Station)
Pre-Gaia 25 pc White Dwarf Sample: New Trigonometric Parallaxes from CTIOPI and NOFS
I present new white dwarf trigonometric parallaxes from both the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) and the U.S. Naval Observatory - Flagstaff Station (NOFS) parallax program. In total, 83 parallaxes are presented and include parallaxes for (1) 27 new systems via CTIOPI, (2) 22 new systems via NOFS, (3) three new systems measured by both programs, (4) one new white dwarf companion to a Hipparcos star, and (5) 30 updated parallaxes for known 25 pc members. I highlight exceptional white dwarfs and discuss the sample as we know it before Gaia, which is expected to largely complete the sample within this volume.

Valentim da Costa Lima, Rodolfo (Instituto de Astronomia, São Paulo), Kepler, S. O., Romero, A. D., Horvath, J. H.
Bayesian evidence of the existence of two populations of white dwarfs: preliminary outcomes
White Dwarfs populations are analysed under the point of view of Bayesian tools. Bayesian Analysis allows inferring possible evolutionary paths through from study masses scales of the White Dwarfs. We analyse the mass distribution of a sample of 2761 DA white dwarf stars from the SDSS, and obtained the central mass values and their corresponding standard deviation. As an input, a bimodal population was proposed for the mass distribution, obtaining as a result a first population with M1 = 0.60 Msun and +/- 0.01 Msun, corresponding to a single stellar evolution, and a second population with M2 = 1.00 Msun and +/- 0.21 Msun possibly due to binary evolution resulting in mergers. The Bayesian approach can be applied study the mass distribution of DA white dwarfs in order to find a possible evolutionary path.