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

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 Gain-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 spec[ral change, with no spectral evolution being ruled out at 5a. 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 Mfi/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.