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

Understand

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 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.