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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