Astronomy and Astrophysics, volume 320, 91-100 (1997/4-1)
Spectral analyses of central stars of planetary nebulae of early WC-type NGC 6751 and Sanduleak 3.
KOESTERKE L. and HAMANN W.-R.
Abstract (from CDS):
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.
stars: Wolf-Rayet - stars: abundances - stars: mass-loss - planetary nebulae: individual: NGC 6751, Sand 3 (WR 72)
Sand 3 = PN Sa 1-3
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