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3D atmospheric models of helium-dominated atmosphere white dwarfs
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Cukanovaite, Elena (2020) 3D atmospheric models of helium-dominated atmosphere white dwarfs. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3518290~S15
Abstract
All stars below around 10M will eventually become white dwarfs, making them the most common type of stellar remnant. Due to the large densities of white dwarfs, their atmospheres are dominated by the lightest element present, with around 80% of white dwarfs in magnitude-limited samples possessing hydrogen-dominated atmospheres. A significant portion of the remaining white dwarfs posses helium-dominated atmospheres, which are the result of born-again or late thermal pulse scenarios, where hydrogen is either completely burned or is diluted during or after the AGB phase. These white dwarfs are the subject of this thesis.
A major uncertainty in the current 1D atmospheric models of white dwarfs lies in the treatment of convective energy transport, usually modelled under the mixing length approximation, which depends on a free parameter called the mixing length parameter, ML2/α. 3D simulations improve upon this by treating convection from first principles and by not relying on any free parameters, resulting in more physical models. In this thesis, I present the first 3D atmospheric models of white dwarfs that posses cool pure-helium atmospheres (DB) and helium-dominated atmospheres with traces of hydrogen (DBA). These models were calculated with the CO5BOLD radiation-hydrodynamics code and cover the hydrogen-to-helium number ratios of −10.0 ≤ log H/He ≤ −2.0, surface gravities of 7.5 ≤ log g ≤ 9.0 and effective temperatures of 12 000 K . Teff . 34 000 K.
To determine the 3D effects on spectroscopic parameters, I compare the synthetic spectra computed from 3D and 1D models. In 1D models, the mixing length parameter is set to a commonly used value of 1.25. The 3D corrections on spectroscopically-derived values of hydrogen abundance and effective temperature are similar in magnitude to typical observational errors. However, the 1D models overestimate the surface gravity for Teff 22 000 K. By increasing hydrogen abundance in the atmosphere, the surface gravity corrections shift to a lower effective temperature range.
To test the 3D spectroscopic corrections, the Sloan Digital Sky Survey (SDSS) spectroscopic sample of DB and DBA white dwarfs is used, alongside the astrometric and photometric data from Gaia data release 2. Both 1D and 3D spectroscopic parameters are found to agree with Gaia within 1-3σ for individual white dwarfs, yet neither type of model produces a perfect agreement.
The uncertainty in line broadening caused by the effect of the neutral helium atom on its own species is also investigated to better understand additional systematic issues in current 1D and 3D model spectra. By comparing several samples of DA and DB/DBA white dwarfs, I show that the precision and accuracy of both types of 3D models are similar.
To extend the usefulness of 3D atmospheric models, I perform the calibration of the mixing length parameter for the bottom of the convection zone in order to determine more accurate bulk properties of the convection zone, such as its mass. Thus, the calibration is applicable for studies of planetary debris around white dwarfs, carbon dredge-up from the core, envelope and astero-seimological models. Overall, the calibrated value of the mixing length parameter is found to be around 0.8 and is much lower than the commonly used value of ML2/α = 1.25 in DB and DBA 1D modelling, meaning that convective efficiency was previously overestimated by a significant factor. This is the first step in investigating convective overshoot in helium-dominated atmosphere white dwarfs.
| Item Type: | Thesis (PhD) | ||||
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| Subjects: | Q Science > QB Astronomy | ||||
| Library of Congress Subject Headings (LCSH): | White dwarf stars, Stars -- Atmospheres, Cosmic abundances | ||||
| Official Date: | September 2020 | ||||
| Dates: |
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| Institution: | University of Warwick | ||||
| Theses Department: | Department of Physics | ||||
| Thesis Type: | PhD | ||||
| Publication Status: | Unpublished | ||||
| Supervisor(s)/Advisor: | Tremblay, Pier-Emmanuel | ||||
| Format of File: | |||||
| Extent: | xxii, 194 leaves : illustrations (some colour) | ||||
| Language: | eng |
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