Hydrogen-deficient stars in pre-WD stages
W.-R. Hamann and G. Gräfener
Among the Central Stars of Planetary Nebulae, about 20% are hydrogen-deficient at their surface (spectral types [WC], wels, PG1159). Because of strong mass-loss, their spectra are formed in a stellar wind and require adequate modelling for their analysis. Corresponding Non-LTE model atmospheres have been developed in the last decade. They have been applied extensively for analyzing the spectra of these hydrogen-deficient white dwarf progenitors, establishing their stellar parameters and surface compositions. The results provide the basis for the discussion of their evolutionary status and have initiated efforts to develop new calculations for the post-AGB evolution which account consistently for diffusive mixing and nuclear burning.
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