Cunningham, T. J. (2020) Convective overshoot in the atmosphere of white dwarf stars. PhD thesis, University of Warwick.

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Abstract

At least a quarter of white dwarf stars have heavy elements in their atmosphere. The source of these metals is predominantly accretion of the evolved planetary systems they hosted as main sequence stars. Precise spectroscopic abundance measurements provide the only method by which to determine the bulk composition of exoplanetary material. As white dwarfs cool they develop surface convection zones and the mass mixed in these regions is a critical input into models of debris-accretion at white dwarfs. But so far all studies have relied upon a century old prescription of convection called the mixing length theory.

In this thesis I employed 3D radiation hydrodynamics (RHD) with the CO5BOLD code to study the properties of convection zones in the surface layers of hydrogen-atmosphere (DA) white dwarfs. I used passive scalars to derive the effective diffusion coefficient due to convective overshoot for white dwarfs in the effective temperature range 11 400–18 000 K. I found that when compared to the mixing length theory the mixed mass is expected to increase by up to 2.5 orders of magnitude. This directly impacts the inferred accreted masses and leads to an order of magnitude increase in inferred accretion rates.

I also present the first transport coefficients in white dwarf atmospheres which describe the advection-diffusion of a passive scalar across the surface. These results cover most of the parameter space of convective white dwarfs and lead to an investigation into whether convection zones are effective at spreading material across the surface. I find that warm (& 14 000 K) DA white dwarfs are unable to efficiently homogenize material at the surface, challenging the often-held assumption that observed metals are homogeneously mixed. For cooler DAs and helium-atmosphere (DB) white dwarfs I find evidence that supports a homogeneous distribution of metals across the surface.

Finally, I analysed a sample of white dwarfs observed with Gaia, SDSS, and Galex to test the hypothesis that DA white dwarfs will transform into DB-type objects when their hydrogen convection zone reaches the deeper helium layer. I found that 22% of white dwarfs will undergo such a spectral change in the temperature range 20 000 K to 9000 K, with no spectral evolution ruled out at 5f. The rate of spectral change was combined with the previous determination of convection zone mass to derive a distribution of hydrogen mass in white dwarfs. These results can serve as an input to asteroseismological models and a constraint on stellar evolution models.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	White dwarf stars, Accretion (Astrophysics), Stars -- Atmospheres, Cosmic abundances
Official Date:	June 2020
Dates:	Date Event June 2020 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Tremblay, Pier-Emmanuel
Sponsors:	European Research Council ; National Science Council (U.S.)
Format of File:	pdf
Extent:	ix, leaves : illustrations, charts
Language:	eng

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