SIMBAD references

Previous investigations have shown that time-independent diffusion calculations cannot explain the large abundances of carbon and oxygen in the helium-dominated atmospheres of hot pre-white dwarfs. Therefore we investigate in this paper the time evolution of the chemical composition in the outer regions with surface layer masses ≤10^–8M_*. The elements He, C, N, O and Ne are taken into account. It is shown that for logg=7.0 and T_eff=140000K and 65000K, respectively, gravitational settling should lead to a strong depletion of both elements within a time-scale of 1000y. According to theoretical evolutionary tracks for post-AGB stars, PG 1159 stars with M_*∼0.6M_☉ should have passed through regions in the HRD with T_eff=140000K and logg∼6.0. For these model parameters the radiative forces are strong enough to transform a helium-rich atmosphere with only traces of heavy elements into a metal-rich one. However, this transformation takes about 800000y, long compared to the corresponding time-scales of stellar evolution. For more massive objects, the observational results cannot be explained by diffusion processes alone.