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Physics Days at Warwick: Summer workshop on white dwarfs and related objects

Date: Monday 5th July 2021 - Tuesday 6th July 2021
SOC: Pier-Emmanuel Tremblay (Warwick), Elena Cukanovaite (Warwick), Chris Manser (Imperial),
Catriona McDonald (Warwick), Tim Cunningham (Warwick) and Mark Hollands (Warwick)

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Part of the Physics Days at Warwick, we invite UK/EU-based researchers (and all international participants who wish to attend) to discuss current and future research on white dwarfs and related objects. This is in the context of the upcoming Gaia Data Release 3 and the follow-up of 100,000s of white dwarfs with multi-object spectroscopic surveys (4MOST/WEAVE/DESI/SDSS-V) and photometric surveys (LSST/ZTF).

Click here for a compact version of the programme.

Monday 5 July, Session 1
Session chair: Pier-Emmanuel Tremblay

9:00-9:05 (BST) / 10:00-10:05 (CEST), Welcome

9:05-9:30 (BST) / 10:05-10:30 (CEST), Martin Barstow:

A Gaia view of the white dwarf population and luminosity function

The ESA Gaia mission data releases are providing us with an unprecedented amount of exquisite astrometric and photometric data. The ability to determine reliable absolute magnitudes, from the parallaxes and photometry, for sources in the Gaia catalogues makes it straightforward to identify large numbers of white dwarfs (WDs) from their location in the Hertzsprung-Russell (H-R) diagram, providing large, high precision statistical samples for detailed study of the properties of the WD population. The availability of a large sample of WDs with known distances presents an opportunity of computing a WD luminosity function (WDLF) of unprecedented precision. We have calculated the WDLF for the WDs in the Gaia Catalogue of Nearby Stars, containing ~20,000 objects, in which several statistically significant step-like features are evident. We present a new version of the WDLF extending to greater depth, beyond 100pc. The increased volume yields a factor ~3 more stars and a concomitant improvement in the statistical errors. We discuss limitations of the sample included in the WDLF and provide a preliminary interpretation of the features observed.

9:30-10:00 (BST) / 10:30-11:00 (CEST), Nicola Gentile Fusillo:

Finding them all: a new catalogue of white dwarfs in Gaia EDR3

The release of Gaia DR2 in 2018 represented a revolution in our understanding of white dwarfs allowing to identify these stellar remnants on an unprecedented scale. These huge white dwarf samples certainly proved to be very fruitful, but they were not without their own limitation. In other words, DR2 was just the start. Gaia EDR3 with its improved parallax and proper motion measurements, represents a new opportunity to expand and refine our search for white dwarfs and explore the properties of these objects from new directions. I will present a new catalogue of white dwarfs based on EDR3 focusing on new features and improvement compared to DR2. I will conclude by highlighting some of the limitation that still exist and giving a quick overview of potential applications of this new sample which now counts nearly 360,000 white dwarfs.

10:00-10:30 (BST) / 11:00-11:30 (CEST), Discussion: Gaia DR3 catalogue and beyond (Barstow, Jordan)

10:30-11:00 (BST) / 11:30-12:00 (CEST), Coffee break (social discussion in Coffee break channel)

Monday 5 July, Session 2
Session chair: Chris Manser

11:00-11:30 (BST) / 12:00-12:30 (CEST), Jack McCleery:

Gaia white dwarfs within 40pc

The Gaia mission has been transformative for many areas of astronomy, with the majority of stars within the local universe having been observed. Despite this drastic increase in size, a full understanding of the local stellar population is still a major challenge. Spectroscopic follow up of nearby white dwarfs is needed to determine precise stellar parameters. The advantages of this are plentiful, from deriving the local stellar formation history to testing white dwarf model accuracy. In my talk, I will discuss the results from a spectroscopic follow-up survey of over 100 white dwarf candidates within the northern hemisphere 40pc sample.

11:30-12:00 (BST) / 12:30-13:00 (CEST), Discussion: White dwarf observations 2021-2026: what are we missing? (Gänsicke)

12:00-13:00 (BST) / 13:00-14:00 (CEST), Lunch break

Monday 5 July, Session 3
Session chair: Chris Manser

13:00-13:30 (BST) / 14:00-14:30 (CEST), Ingrid Pelisoli:

Uncovering white dwarfs that only binary evolution can explain

Single stellar evolution cannot explain all the features in the Gaia colour-magnitude diagram. That is not surprising: most stars with one solar mass or above are part of a binary system, and around a quarter of those will interact during their lifetime, giving rise to astrophysical phenomena not witnessed by single stars. One particularly interesting scenario occurs when interaction happens as a star is leaving the main sequence before the onset of core helium burning. This can lead to enhanced mass-loss that allows the formation of so-called extremely-low mass white dwarf stars (ELMs). In this talk, I will present efforts towards a Gaia catalogue of ELM candidates, as well as preliminary results from a volume-limited survey of these objects.

13:30-14:00 (BST) / 14:30-15:00 (CEST), Maria Georganti:

Magnetic propeller in low-accretion rate dwarf novae: the case of WZ Sge

The lowest accretion rate dwarf novae exhibit a complex light curve morphology consisting of a sharp, deep “dip” and subsequent multiple “echo-outbursts” on their super-outburst decline. As the standard disk instability model fails to predict this peculiar decline-phase behaviour, we investigate that these abrupt changes between faint and bright states represent transitions into and out of a magnetic propeller state. In this talk, I will present the results of our search for a propeller signature in time-resolved ultraviolet spectroscopy taken just before, during and after the dip in WZ Sge’s 2001 super-outburst.

14:00-14:30 (BST) / 15:00-15:30 (CEST), Discussion: Unresolved WD+MS binaries in Gaia eDR3 (Parsons, Rebassa Mansergas)

14:30-15:00 (BST) / 15:30-16:00 (CEST), Coffee break (social discussion in Coffee break channel)

Monday 5 July, Session 4
Session chair: Tim Cunningham

15:00-15:30 (BST) / 16:00-16:30 (CEST), Nicole Reindl:

Mysterious, variable, and extremely hot: a new class of variable white dwarfs uncovered

About 10% of all stars exhibit absorption lines of ultra-highly excited (UHE) metals (e.g. O VIII) in their optical spectra when entering the white dwarf cooling sequence. Here I will present our recent results from a photometric variability study of all known UHE white dwarfs, and discuss the implications of our findings on our understanding of these intriguing objects.

15:30-16:00 (BST) / 16:30-17:00 (CEST), Stella Stopkowicz and Mark Hollands:

Modelling the spectra of highly magnetised DZ white dwarfs

Magnetic white dwarfs are ideal laboratories for testing atomic physics under extreme magnetic fields. In the last 20 years, around 50 metal-rich magnetic white dwarfs have been discovered, with field strengths up to ~10MG. We have identified a metal-rich white dwarf with a complicated spectrum indicative of a much higher field. Using state-of-the-art finite-field coupled-cluster methods, we have calculated transition wavelengths and oscillator strengths for Na and Mg for field strengths up to 1000MG. These calculations reveal a mere 30MG field for our object, and pave the way for classifying unusual spectra in future spectroscopic surveys.

16:00-16:30 (BST) / 17:00-17:30 (CEST), Andrew Buchan:

Exoplanet geochemistry as a method for constraining the masses of white dwarf pollutant bodies

Polluted white dwarfs which have accreted fragments of rocky material provide a unique opportunity to probe the interiors of exoplanetary bodies. The compositions of such bodies encode information about their formation histories, including the geological process of core-mantle differentiation. The behaviour of Cr, Ni, and Si during differentiation is affected by the pressure within the differentiating body. This alters the composition of the resulting core and mantle material. In my talk, I will present a model which uses these geochemical signatures to constrain the pressure, and hence size, of the bodies which pollute white dwarfs.

16:30-17:00 (BST) / 17:30-18:00 (CEST), Christian Knigge:

Discovery of a Distant Proper Motion Companion to the (In)famous Nova T Pyxidis: A Triple Star Origin For Short-Period Recurrent Novae

Recurrent novae are star systems in which a massive white dwarf accretes material at such a high rate that it undergoes thermonuclear runaways every 1 - 100 years. They are the only class of novae in which the white dwarf can grow in mass, making some of these systems strong Type Ia supernova progenitor candidates. Almost all known recurrent novae are long-period (P > 12 hrs) binary systems in which the requisite mass supply rate can be provided by an evolved (sub-)giant donor star. However, at least two recurrent novae are short-period (P < 3 hrs) binaries, in which mass transfer would normally be driven by gravitational radiation at rates 3-4 orders of magnitude smaller than required. In this talk, I will show that the prototype of this class -- T Pyxidis -- has a distant proper motion companion in Gaia EDR3 and therefore likely evolved from a hierarchical triple star system. Triple evolution can naturally produce exotic compact binaries as a result of three-body dynamics, either by Kozai-Lidov eccentricity cycles in dynamically stable systems or via mass-loss-induced dynamical instability. I will present numerical simulations in which triple progenitors with physically reasonable parameters are evolved forward in time, which show explicitly that the inner binary can become so eccentric that mass transfer is triggered at periastron, driving the secondary out of thermal equilibrium. Short-period recurrent novae likely evolved via this extreme state, explaining their departure from standard binary evolution tracks.

Tuesday, 6 July Session 5
Session chair: Catriona McDonald

09:00-09:30 (BST) / 10:00-10:30 (CEST), Roberto Raddi:

Kinematic properties of white dwarfs

We present the detailed kinematic analysis of a sample of about 3100 single white dwarfs, which have available radial velocity measurements, and common proper motion pairs that contain a white dwarf and a non-degenerate companion, for which radial velocities and chemical composition have been determined by large scale spectroscopic surveys. Combining the radial velocities with accurate Gaia astrometry and proper motions, we derive the velocity components of our sample in a Galactic rest frame. The age velocity-dispersion of the samples are discussed, along with the correlations among metallicity and kinematics of a few interesting systems.

09:30-10:00 (BST) / 10:30-11:00 (CEST), Carlos López San Juan:

Spectral evolution and metal-polluted white dwarfs with multi-filter data: J-PLUS DR2 and mini-JPAS

We have complemented the WD Gaia catalogue with the multi-filter photometry from J-PLUS DR2 (12 optical filters, 2200 deg², 5800 WDs) and mini-JPAS (56 optical narrow filters, 1 deg², 11 WDs). We summarize our results on (1) spectral evolution with effective temperature from J-PLUS, finding an increase of He-dominated WDs at lower Teff; (2) the search for metal-polluted WDs using the J0395 filter in J-PLUS, providing candidates and estimating the fraction of calcium WDs with Teff; (3) WD science forecast for J-PAS (R ~ 50) and Gaia DR3 spectrophotometry (R ~ 30-90) using mini-JPAS data.

10:00-10:30 (BST) / 11:00-11:30 (CEST), Laura Rogers:

An observational perspective on polluted white dwarf systems

Around a third of single white dwarfs are externally polluted by planetary material. It is hypothesised that chunks of planetary material get perturbed onto star grazing orbits, tidally disrupt, and accrete onto the white dwarfs' atmosphere. I will give an observational perspective on polluted white dwarf systems, discussing the circumstellar gas and dust, which eventually accretes onto the white dwarf, and the bulk composition of the accreted planetary material.

10:30-11:00 (BST) / 11:30-12:00 (CEST), Coffee break (Coffee break channel)

Tuesday 6 July, Session 6
Session chair: Catriona McDonald

11:00-11:30 (BST) / 12:00-12:30 (CEST), Marc Brouwers:

A three-stage model for white dwarf pollution: Tidal disruption, grind-down and rapid circularisation

We propose a universally applicable, three-stage model for the accretion of planetary material onto white dwarfs. In the first stage, an asteroid tidally disrupts around the star to form a highly eccentric tidal disc. In the second stage, differential precession and other processes induce high-velocity collisions between fragments that grind them into dust. The third component of our model is the rapid circularisation and accretion of the grains that are produced. We suggest that PR drag alone cannot drive sufficiently rapid dust accretion and that gas or magnetic fields likely play a key role in this final stage.

11:30-12:00 (BST) / 12:30-13:00 (CEST), Discussion: The missing link: where should we look for the final clues to complete our understanding of how planetary material arrives in the atmospheres of white dwarfs (Bonsor)

Microsoft Teams link to the meeting to be sent in final announcement email (click here for late registration).

This meeting is supported by the European Research Council (project WD3D)