On the Spectral Evolution of Hot White Dwarf Stars. IV. The Diffusion and Mixing of Residual Hydrogen in Helium-rich White Dwarfs

Abstract In the framework of our extensive modeling study spectral evolution white dwarfs, we present here a new set detailed calculations transport residual hydrogen in helium-rich dwarfs. First, investigate so-called float-up process at high effective temperatures, whereby upward diffusion trace leads to formation atmosphere. We examine dependence this phenomenon on initial abundance and strength radiative wind that opposes gravitational settling. Combined with empirical knowledge evolution, simulations provide quantitative constraints content hot helium-dominated dwarf population. Then, outcome convective dilution low superficial layer is mixed within underlying envelope. stark contrast previous works dilution, demonstrate that, under reasonable assumptions, models successfully reproduce observed atmospheric composition cool DBA stars, thereby solving one most important problems theory evolution. This major improvement due self-consistent earlier process, which predicts existence massive reservoir underneath thin layer. argue detected surface dwarfs is, cases, primordial origin rather than result external accretion.