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Astron. Astrophys. 320, 91-100 (1997)

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Spectral analyses of central stars of planetary nebulae of early WC-type *

NGC 6751 and Sanduleak 3

L. Koesterke and W.-R. Hamann

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

Received 7 April 1996 / Accepted 12 September 1996

Abstract

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 [FORMULA]. 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 [FORMULA], defined as the effective temperature related to the stellar core radius, is about 140 kK for both stars. Adopting a luminosity of 5000 [FORMULA], the stellar radii and mass-loss rates are determined (NGC 6751: 0.13 [FORMULA], [FORMULA]... [FORMULA] [FORMULA], Sand 3: 0.12 [FORMULA], [FORMULA] [FORMULA]). 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 [FORMULA] WC [FORMULA] stars. The results of our analyses are in rough accordance with the "born-again scenario" and suggest an evolutionary path [FORMULA] WC-late [FORMULA] WC-early [FORMULA] PG 1159. In conflict is the carbon abundance, as we derive lower values than found in [FORMULA] WC-late [FORMULA] and PG 1159 stars.

Key words: stars: Wolf-Rayet – stars: abundances – stars: mass-loss – planetary nebulae: individual: NGC 6751, Sand 3

* Partly based on observations obtained at the German-Spanish Astronomical Center, Calar Alto, Spain

Send offprint requests to: L. Koesterke

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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