1997A&A...320...91K

The two Wolf-Rayet type central stars of the planetary nebulae NGC 6751 and Sanduleak 3, showing the early spectral subtypes [WC4] and [WC2-3], respectively, are analyzed by means of non-LTE models for spherically expanding atmospheres. Complex model atoms of helium, carbon, oxygen and nitrogen are taken into account. For the atmospheric composition we find a carbon-to-helium mass ratio of about 0.5 in both stars. The oxygen mass fraction is about 0.15±0.1. The nitrogen abundance in Sand 3 is about 0.005. In NGC 6751 nitrogen is also clearly present but a quantitative determination failed. The parameter T_*, defined as the effective temperature related to the stellar core radius, is about 140kK for both stars. Adopting a luminosity of 5000L_☉, the stellar radii and mass-loss rates are determined (NGC 6751: 0.13R_☉, 10^–6.1 ... 10^–5.5M_☉/yr, Sand 3: 0.12R_☉, 10^–6.15M_☉/yr). From the spread of ionization stages in the optical spectrum of NGC 6751 we find hints for inhomogeneities in the atmosphere and estimate a density contrast of 0.6dex. The stellar evolution is discussed with regard to recently analyzed PG1159 and late-type [WC] stars. The results of our analyses are in rough accordance with the "born-again scenario" and suggest an evolutionary path [WC-late]->[WC-early]-> PG1159. In conflict is the carbon abundance, as we derive lower values than found in [WC-late] and PG1159 stars.