1996A&A...306..843U

The problem under discussion is: Can the large carbon and oxygen abundances observed in the atmospheres in PG 1159 stars be explained by the effects of gravitational settling and selective radiative forces? With a new iteration method the radiative transfer equation and the momentum balance equations for the elements He, C, N, O and Ne are solved simultaneously. A detailed computation of continuum opacities for the various elements allows to take into account the feedback effects between chemical composition and temperature structure. The chemical composition in the atmosphere can be predicted in a definite way if the C-O core is surrounded by a thick helium mantle according to standard stellar evolution theory. A comparison with the observational results shows that the predicted metal abundances are too low for two objects with surface gravities logg≥7 and too large for objects with logg≤6. Provided that diffusion time scales are short compared to the time scales of stellar evolution this implies that gravitational settling and the effect of selective radiative forces cannot be the only relevant physical mechanisms working in the atmospheres of PG 1159 stars. The possibility is discussed that the elements are enriched by diffusion during the post-AGB evolution. Whereas the levitation of heavy elements in the atmospheres of cooler white dwarfs is brought about by the momentum transferred from photons to matter via bound-bound transitions, the momentum transferred via bound-free transitions is responsible for the large enhancement of heavy elements in the atmospheres of stars with T_eff≥100000K and logg≤6.