2000A&A...359.1042U

We present the results of new diffusion calculations with mass loss for hot white dwarfs with T_eff>50000K. In contrast to previous calculations, the stellar parameters T_eff, logg and the mass loss are allowed to vary. With stellar parameters from evolutionary tracks and two independent estimates of the mass loss rates the time-dependance of the composition during the cooling is investigated in an outer region of about 10^–4M_*. With an improved numerical method the elements H, He, C, N and O can be taken into account simultaneously. In the T_eff-logg -diagram we expect a DAO/DA transition region near an approximately horizontal line characterized by logg≃7. In hydrogen-rich objects with lower surface gravities helium lines should be detectable, because mass loss with {dot}(M)≥10^–12M_*/y prevents or retards the gravitational settling of helium. PG 1159 stars are predicted to evolve into DO's when the surface gravity reaches a value between logg=7.5 and 8.0 and the mass loss rate decreases below about 10^–13M_*/y. From the results we suggest the existence of an evolutionary link between PG 1159 stars and most of the DO's. Calculations with various admixtures of hydrogen in PG 1159 stars show that most DAO's cannot be descendants of hydrogen-poor PG 1159 stars with a number ratio H/He≤0.1, because mass loss is expected to retard the floating up of hydrogen. The majority of hot DAO's and DA's must be descendants of hydrogen-rich precursors.