Spectral analyses of central stars of planetary nebulae of early WC-type NGC 6751 and Sanduleak 3.
Abstract
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 5000Lsun_, the stellar radii and mass-loss rates are determined (NGC 6751: 0.13Rsun_, 10^-6.1^ ... 10^-5.5^Msun_/yr, Sand 3: 0.12Rsun_, 10^-6.15^Msun_/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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1997
- Bibcode:
- 1997A&A...320...91K
- Keywords:
-
- STARS: WOLF-RAYET;
- STARS: ABUNDANCES;
- STARS: MASS-LOSS;
- PLANETARY NEBULAE: INDIVIDUAL: NGC 6751;
- SAND 3