Spectral analyses of central stars of planetary nebulae of early WC-type
NGC 6751 and Sanduleak 3

L. Koesterke & W.-R. Hamann

Lehrstuhl Astrophysik, Universität Potsdam
Am Neuen Palais 10, D-14469 Potsdam, Germany
e-mail (L.K.): lars@astro.physik.uni-potsdam.de

The two Wolf-Rayet type central stars of the planetary nebulae NGC 6751 and Sanduleak 3, showing the early spectral subtypes [WC 4] and [WC 2-3], 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 Tstar, defined as the effective temperature related to the stellar core radius, is about 140kK for both stars. Adopting a luminosity of 5000Lsun, the stellar radii and mass-loss rates are determined (NGC 6751: 0.13 Rsun, 10-6.1 ... 10-5.5 Msunyr-1, Sand 3: 0.12 Rsun, 10-6.15 Msunyr-1). 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 0.6 dex. The stellar evolution is discussed with regard to recently analyzed PG 1159 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] -> PG 1159. In conflict is the carbon abundance, as we derive lower values than found in [WC-late] and PG 1159 stars.

Keywords: stars: Wolf-Rayet -- stars: atmospheres -- stars: abundances -- stars: mass-loss -- stars: individual: NGC 6751, Sand 3

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