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Evolving extrasolar Kirkwood gaps

Dimitri Veras, Nikolaos Georgakarakos, Ian Dobbs-Dixon

The evolution of planetary debris discs, such as extrasolar analogues of the solar system's Main Belt and Kuiper Belt, crucially determine how host stars are polluted with planetary metals. This new paper presents high-resolution resonant portraits of these debris discs as they evolve beyond the main-sequence and interact with a surviving planet in the system. We find that evolving resonant structure can be accurately bound with only main-sequence values by computing a maximum libration width as a function of asteroid longitude of pericentre. We also quantify the relative efficiency of mean motion resonances of different orders to stabilize versus destabilize asteroid orbits during both the giant branch and white dwarf phases. The 4:1, 3:1 and 2:1 resonances represent efficient polluters of white dwarfs, and even when in the orbit-crossing regime, both the 4:3 and 3:2 resonances can retain small reservoirs of asteroids in stable orbits throughout giant branch and white dwarf evolution.

Article

Tue 08 Nov 2022, 14:35 | Tags: Debris Discs

RAS Specialist Meeting - Formation, Evolution and Dispersal of Protoplanetary Discs

Amena Faruqi -- Simulating the Transit of a Binary System by a Circumbinary Disc in the HD98800 System

The properties of circumbinary discs play a significant role in star-disc interactions in binary systems. However, there are gaps in our current understanding of how various disc properties, such as the disc mass, viscosity, and dust-to-gas ratio, can affect the dynamics and evolution of such a system. In order to better understand the influence of these properties, it would be ideal to study a system where the stellar parameters are well-constrained, and the disc can be imaged. One such system is the HD98800 system, a quadruple star system consisting of two binaries, AaAb and BaBb, one of which (BaBb) has been observed to have a circumbinary protoplanetary disc encircling it. In 2026, it has been predicted that the circumbinary disc of BaBb will pass in front of AaAb. This provides a rare opportunity – analysing the light curve of the system during this transit could allow us to constrain certain properties of the disc. The goal of this research is to inform future observations and subsequent study of the system and transit, as well as providing some insight into the properties of circumbinary protoplanetary discs.

 

In this presentation, I will discuss my research using hydrodynamical simulations and radiative transfer modelling to create synthetic observations of the HD98800 system and anticipated transit. To consider how observations differ as a result of variations in disc properties, a grid of models defined by a parameter space was created. I will discuss how this set of results can be compared to identify features in the synthetic light curves that can be used to make inferences about disc properties. I will also consider how my synthetic observations compare to past observations of the system and more broadly, how certain disc properties can be constrained using simulation data.

Fri 14 Oct 2022, 14:20 | Tags: Talk/Poster Protoplanetary Discs

Debris Discs: Home and Abroad 2022

Raphael Bendahan-West -- Automated Search for Spectroscopic Exocomet Transits

Exocomets are analogues of solar system comets, with the main difference being that they orbit stars other than the Sun. These extrasolar comets can be thought of as small icy planetesimals that were left unused during the planet formation phase of a system. Although being “leftover” dust agglomerates, exocomets tend to be part of the most pristine objects in a system, enclosing primordial information about the early phases of planetary formation.

The field of exocomets has been built around the unmatched number of detections made in the circumstellar disc of the archetypal star Beta Pictoris. An exocomet detection is identified by variable absorption features around atomic lines in a stellar spectrum (typically around the ionised Calcium doublet), which can be explained by the interaction between the light emitted by a star and the transiting gas that is in and trailing an exocomet coma. Variability in the CaII doublet has been observed around approximately 30 different stars – certain detections being clearer than others, emphasising the rarity and difficulty to observe such events.

In this talk, I will show results from an automated search through the HARPS archive for signs of exocomet transits. Such a spectroscopic survey is unprecedented mainly due to the large quantity and wide variety of stars being observed. This talk will introduce our newly developed automated search algorithm that quantifies the rate of CaII exocomet absorption events in archival spectra, with the goal to estimate the occurrence rate of transiting exocomets.

Azib Norazman -- An Automated Search for Transiting Exocomets with TESS

The term exocomets refers to analogues of our Solar System comets. They are small icy planetesimal bodies that orbit stars other than the Sun and are thought to be the remnants of exoplanet formation. In recent years, there have been a small number of stars that have unique asymmetric dips in their transits from photometric data that cannot be caused by a planet transit. The most prominent example of this so far are from the lightcurves of Beta Pictoris, a young A-type star. Recent literature has also indicated that exocomet detections are more likely in younger, A/F-type stars. As the TESS mission carries out an all-sky survey, there is now the opportunity to explore this hypothesis further and explore the detection rates of exocomets with relation to the spectral type of their host star- uncovering the occurrence rates as a function of spectral type and stellar age. A simple automated method to detect exocomet transits was developed for the Kepler mission. As TESS expands the sample of stars over the whole sky, a plethora of star systems will be explored. However, the nature of the all-sky search results in a wide range of astrophysical variability in addition to the different systematics the spacecraft has compared to Kepler. Therefore, development of the search method for TESS is essential. We present the automated search algorithm used to find potential asymmetric transit dips in TESS, and the results of my search at the time of the conference.

Mon 29 Aug 2022, 09:20 | Tags: Talk/Poster Exocomets Debris Discs

Rocking shadows in protoplanetary discs

Rebecca Nealon, Daniel Price and Christophe Pinte

Numerical simulation of a protoplanetary disc accompanied by synthetic scattered light images showing shadows

Astronomers from the University of Warwick reveal a new phenomenon dubbed the “rocking shadow” effect that describes how discs in forming planetary systems are oriented, and how they move around their host star. The effect also gives clues as to how they might evolve with time. Dr Rebecca Nealon presented the new work this week at the 2022 National Astronomy Meeting at the University of Warwick.

Article Press Release

Thu 28 Jul 2022, 09:05 | Tags: Protoplanetary Discs

ALMA's view of the M-dwarf GSC 07396-00759's edge-on debris disc: AU Mic's coeval twin

Patrick F. Cronin-Coltsmann, Grant M. Kennedy, Christian Adam, Quentin Kral, Jean-François Lestrade, Sebastian Marino, Luca Matrà, Simon J. Murphy, Johan Olofsson, Mark C. Wyatt

We present new ALMA Band 7 observations of the edge-on debris disc around the M1V star GSC 07396-00759. At ~20 Myr old and in the beta Pictoris Moving Group along with AU Mic, GSC 07396-00759 joins it in the handful of low mass M-dwarf discs to be resolved in the sub-mm. With previous VLT/SPHERE scattered light observations we present a multi-wavelength view of the dust distribution within the system under the effects of stellar wind forces. We find the mm dust grains to be well described by a Gaussian torus at 70 au with a FWHM of 48 au and we do not detect the presence of CO in the system. Our ALMA model radius is significantly smaller than the radius derived from polarimetric scattered light observations, implying complex behaviour in the scattering phase function. The brightness asymmetry in the disc observed in scattered light is not recovered in the ALMA observations, implying that the physical mechanism only affects smaller grain sizes. High resolution follow-up observations of the system would allow investigation into its unique dust features as well as provide a true coeval comparison for its smaller sibling AU Mic, singularly well observed amongst M-dwarfs systems.

Fri 25 Feb 2022, 00:00 | Tags: Debris Discs

Warping Away Gravitational Instabilities in Protoplanetary Discs

Sahl Rowther, Rebecca Nealon, Farzana Meru
Evolution of a Warped GI disc

We perform 3D SPH simulations of warped, non-coplanar gravitationally unstable discs to show that as the warp propagates through the self-gravitating disc, it heats up the disc rendering it gravitationally stable. Thus losing their spiral structure and appearing completely axisymmetric. In their youth, protoplanetary discs are expected to be massive and self-gravitating, which results in non-axisymmetric spiral structures. However recent observations of young protoplanetary discs with ALMA have revealed that discs with large-scale spiral structure are rarely observed in the midplane. Instead, axisymmetric discs with some also having ring & gap structures are more commonly observed. Our work involving warps, non-coplanar disc structures that are expected to commonly occur in young discs, potentially resolves this discrepancy between observations and theoretical predictions. We demonstrate that they are able to suppress the large-scale spiral structure of self-gravitating protoplanetary discs.

Thu 03 Feb 2022, 11:00 | Tags: Protoplanetary Discs

Orbit decay of 2-100 au planetary remnants around white dwarfs with no gravitational assistance from planets

Dimitri Veras, Yusuf Birader, Uwais Zaman

planet migration

Debris discs orbiting white dwarfs are the primary conduit though which planetary remnants accrete onto the photosphere. A widely-held assumption is that these discs are formed by the destruction of asteroids flung towards the white dwarf by a terrestrial or giant planet. However, these planets could have been previously been engulfed by the star or escaped the system, leaving behind asteroids, boulders, cobbles, pebbles, sand and dust. These remaining small bodies could then persist throughout the host star's evolution into a white dwarf at ~2-100 au scales, and then be radiatively dragged towards the white dwarf to create the disc without the help of a planet. Here we identify the parameter space and cooling ages for which this one metal-pollution mechanism is feasible by, for the first time, coupling Poynting-Robertson drag, the Yarkovsky effect and the YORP effect solely from rapidly dimming white dwarf radiation (see figure). We find that this no-planet pollution scenario is efficient for remnant 10^-5-10^-4 m dust up to about 80 au, 10-4-10-3 m sand up to about 25 au and 10^-3-10^-2 m small pebbles up to about 8 au, and perhaps 10^-1-10^0 m small boulders up to tens of au. Further, young white dwarf radiation can spin up large strength-less boulders with radii 10^2-10^3 m to destruction, breaking them down into smaller fragments which then can be dragged towards the white dwarf. Our work hence introduces a planet-less metal-pollution mechanism that may be active in some fraction of white dwarf planetary systems..

Article

Fri 26 Nov 2021, 00:00 | Tags: White Dwarf Discs

Velocity-imaging the rapidly precessing planetary disc around the white dwarf HE 1349-2305 using Doppler tomography

Christopher J. Manser, Erik Dennihy, Boris T. Gänsicke, John H. Debes, Nicola P. Gentile Fusillo, J. J. Hermes, Mark Hollands, Paula Izquierdo, B. C. Kaiser, T. R. Marsh, Joshua S. Reding, Pablo Rodríguez-Gil, Dimitri Veras, David J. Wilson

The presence of planetary material in white dwarf atmospheres, thought to be accreted from a dusty debris disc produced via the tidal disruption of a planetesimal, is common. Approximately five per cent of these discs host a co-orbital gaseous component detectable via emission from atomic transitions - usually the 8600 Angstrom CaII triplet. These emission profiles can be highly variable in both morphology and strength. Furthermore, the morphological variations in a few systems have been shown to be periodic, likely produced by an apsidally precessing asymmetric disc. Of the known gaseous debris discs, that around HE1349-2305 has the most rapidly evolving emission line morphology, and we present updated spectroscopy of the CaII triplet of this system. The additional observations show that the emission line morphologies vary periodically and consistently, and we constrain the period to two aliases of 459±3d and 502±3d. We produce images of the CaII triplet emission from the disc in velocity space using Doppler tomography - only the second such imaging of a white dwarf debris disc. We suggest that the asymmetric nature of these velocity images is generated by gas moving on eccentric orbits with radially-dependent excitation conditions via photo-ionisation from the white dwarf. We also obtained short-cadence (~ 4 min) spectroscopy to search for variability on the time-scale of the disc's orbital period (~ hours) due to the presence of a planetesimal, and rule out variability at a level of ~ 1.4 per cent.

Article

Mon 11 Oct 2021, 00:00 | Tags: White Dwarf Discs

White dwarf planetary debris dependence on physical structure distributions within asteroid belts

Catriona H. McDonald, Dimitri Veras

White dwarfs which exhibit transit signatures of planetary debris and accreted planetary material provide exceptional opportunities to probe the material composition and dynamical structure of planetary systems. Although previous theoretical work investigating the role of minor body disruption around white dwarfs has focussed on spherical bodies, Solar System asteroids can be more accurately modelled as triaxial ellipsoids. Here we present an analytical framework to identify the type of disruption (tidal fragmentation, total sublimation or direct impact) experienced by triaxial asteroids approaching white dwarfs on extremely eccentric (e ~ 1) orbits. This framework is then used to identify the outcomes for simplified Main belt analogues of 100 bodies across five different white dwarf temperatures. We also present an empirical relationship between cooling age and effective temperature for both DA and DB white dwarfs to identify the age of the white dwarfs considered here. We find that using a purely spherical shape model can underestimate the physical size and radial distance at which an asteroid is subjected to complete sublimation, and these differences increase with greater elongation of the body. Contrastingly, fragmentation always occurs in the largest semi-axis of a body and so can be modelled by a sphere of that radius. Both fragmentation and sublimation are greatly affected by the body's material composition, and hence by the composition of their progenitor asteroid belts. The white dwarf temperature, and hence cooling age, can affect the expected debris distribution: higher temperatures sublimate large elongated asteroids, and cooler temperatures accommodate more direct impacts.

Article

Tue 06 Jul 2021, 00:00 | Tags: White Dwarf Discs

A review: planetary systems around white dwarfs

Dimitri Veras

White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from au-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and are accreted onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.

Article

Fri 11 Jun 2021, 18:00 | Tags: White Dwarf Discs

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