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From hydrogen to helium : the spectral evolution of white dwarfs as evidence for convective mixing

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Cunningham, T. J., Tremblay, Pier-Emmanuel, Gentile Fusillo, Nicola Pietro, Hollands, Mark A. and Cukanovaite, Elena (2020) From hydrogen to helium : the spectral evolution of white dwarfs as evidence for convective mixing. Monthly Notices of the Royal Astronomical Society, 492 (3). pp. 3540-3552. doi:10.1093/mnras/stz3638 ISSN 1365-2966.

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Official URL: https://doi.org/10.1093/mnras/stz3638

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Abstract

We present a study of the hypothesis that white dwarfs undergo a spectral change from hydrogen- to helium-dominated atmospheres using a volume-limited photometric sample drawn from the Gaia-DR2 catalogue, the Sloan Digital Sky Survey (SDSS), and the Galaxy Evolution Explorer (GALEX). We exploit the strength of the Balmer jump in hydrogen-atmosphere DA white dwarfs to separate them from helium-dominated objects in SDSS colour space. Across the effective temperature range from 20 000 to 9000 K, we find that 22 per cent of white dwarfs will undergo a spectral change, with no spectral evolution being ruled out at 5σ. The most likely explanation is that the increase in He-rich objects is caused by the convective mixing of DA stars with thin hydrogen layers, in which helium is dredged up from deeper layers by a surface hydrogen convection zone. The rate of change in the fraction of He-rich objects as a function of temperature, coupled with a recent grid of 3D radiation-hydrodynamic simulations of convective DA white dwarfs – which include the full overshoot region – lead to a discussion on the distribution of total hydrogen mass in white dwarfs. We find that 60 per cent of white dwarfs must have a hydrogen mass larger than MH/MWD = 10−10, another 25 per cent have masses in the range MH/MWD = 10−14–10−10, and 15 per cent have less hydrogen than MH/MWD = 10−14. These results have implications for white dwarf asteroseismology, stellar evolution through the asymptotic giant branch and accretion of planetesimals on to white dwarfs.

Item Type: Journal Article
Divisions: Faculty of Science, Engineering and Medicine > Science > Physics
SWORD Depositor: Library Publications Router
Journal or Publication Title: Monthly Notices of the Royal Astronomical Society
Publisher: Oxford University Press (OUP)
ISSN: 1365-2966
Official Date: March 2020
Dates:
DateEvent
March 2020Published
21 January 2020Available
17 December 2019Accepted
Volume: 492
Number: 3
Page Range: pp. 3540-3552
DOI: 10.1093/mnras/stz3638
Status: Peer Reviewed
Publication Status: Published
Access rights to Published version: Open Access (Creative Commons)

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