On the DB gap of white dwarf evolution: effects of hydrogen mass fraction and convective overshooting

Abstract

We investigate the spectral evolution of white dwarfs by considering the effects of hydrogen mass in the atmosphere and convective overshooting above the convection zone. Our numerical results show that white dwarfs with MH ∼ 10−16 M⊙ show the DA spectral type between 46000 ⪅ Teff ⪅ 26000 K and the DO or DB spectral type may appear on either side of this temperature range. White dwarfs with MH ∼ 10−15 M⊙ appear as DA stars until they cool to Teff ∼ 31000 K; from then on they will evolve into DB white dwarfs as a result of convective mixing. If MH in the white dwarfs is more than 10−14 M⊙, the convective mixing will not occur when Teff > 20000 K, thus these white dwarfs always appear as DA stars. White dwarfs within the temperature range 46000 ⪅ Teff ⪅ 31000 K always show the DA spectral type, which coincides with the DB gap. We notice the importance of the convective overshooting and suggest that the overshooting length should be proportional to the thickness of the convection zone to better fit the observations.