Swan, Andrew;
(2022)
Metal pollution and dust at white dwarfs.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
White dwarfs host planetary systems that are unseen, but nevertheless convey a surprising amount of information about their nature. Photospheric metal pollution reflects the bulk composition of minor planets that have experienced destructive encounters with their parent stars, while infrared emission from circumstellar debris reveals details of those interactions. This thesis is an observational study of these systems. Optical spectroscopy of nine polluted white dwarfs is presented and interpreted using a new analysis method, accompanied by a discussion of the limitations of such studies. Sufficient information is available at one star for the exoplanetary material to be modelled as a core–mantle–crust mixture based on Earth abundances. Others show unusual chemistry such as Ca depletion, and a surprising range in Na abundances is found. The results highlight the diversity among individuals, while supporting the consensus that white dwarfs typically accrete material from rocky, volatile-poor objects with origins in differentiated bodies. At least one star in the sample is observed several Myr after the latest major accretion event, and the implications of such observations are discussed. Infrared photometry from the WISE and Spitzer spacecraft are used to probe variability at white dwarfs with detectable debris discs, using archival data spanning more than a decade, as well as targeted observations. Variation is found to be ubiquitous across the population, on timescales of months to decades, with the highest flux changes (tens of per cent) observed at systems where gaseous debris has also been detected. These findings overturn the canonical model of circumstellar debris in geometrically thin, optically thick discs that quiescently feed accretion. While such discs may yet exist, optically thin dust is required to account for the observed variation, and most likely is produced in collisions.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Metal pollution and dust at white dwarfs |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2022. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy UCL > Provost and Vice Provost Offices > UCL BEAMS UCL |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10143418 |
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