From Hydrogen to Helium: The Spectral Evolution of White Dwarfs as Evidence for Convective Mixing

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 $GalaxyEvolutionExplorer$ (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 20000K to 9000K we find that 22% of white dwarfs will undergo a spectral change, with no spectral evolution being ruled out at 5$\sigma$. 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% of white dwarfs must have a hydrogen mass larger than $M_{\mathrm{H}}/M_{\mathrm{WD}}=10^{-10}$, another 25% have masses in the range $M_{\mathrm{H}}/M_{\mathrm{WD}}=10^{-14}-10^{-10}$, and 15% have less hydrogen than $M_{\mathrm{H}}/M_{\mathrm{WD}}=10^{-14}$. These results have implications for white dwarf asteroseismology, stellar evolution through the asymptotic giant branch (AGB) and accretion of planetesimals onto white dwarfs.

14 pages, 13 figures, accepted for publication in MNRAS

References in corpus (16)

• The Gaia mission
• The frequency of planetary debris around young white dwarfs
• The Chemical Composition of an Extrasolar Minor Planet
• On the Measurement of Fundamental Parameters of White Dwarfs in the Gaia Era
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14
• Likely detection of water-rich asteroid debris in a metal-polluted white dwarf
• White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
• Cool DZ white dwarfs I: Identification and spectral analysis
• The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
• White Dwarf Cosmochronology in the Solar Neighborhood
• A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs
• Convective Overshoot and Macroscopic Diffusion in Pure-Hydrogen Atmosphere White Dwarfs
• The Gaia DR1 Mass-Radius Relation for White Dwarfs
• The white dwarf mass-radius relation and its dependence on the hydrogen envelope
• Calibration of the mixing length theory for structures of helium-dominated atmosphere white dwarfs
• Probing the structure of Kepler ZZ Ceti stars with full evolutionary models-based asteroseismology

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• White dwarf pollution by hydrated planetary remnants: Hydrogen and Metals in WD J204713.76-125908.9
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• On the Spectral Evolution of Hot White Dwarf Stars. IV. The Diffusion and Mixing of Residual Hydrogen in Helium-rich White Dwarfs
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• The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX
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• A Random Forest spectral classification of the Gaia 500-pc white dwarf population
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