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Analysis of hydrogen rich PG 1159 central stars of Planetary Nebulae

  • S. Dreizler (a1) (a2), K. Werner (a1) (a2) and U. Heber (a1) (a2)

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The PG 1159 stars represent the hottest stage of post-AGB evolution. Quantitative spectral analyses of most known PG 1159 stars have been carried out by us from optical, UV and EUV observations (see Dreizler et al 1995 for a review). It has been shown that these stars have atmospheres dominated by C and He with a significant admixture of O. These abundances reveal the inter–shell matter of a former AGB star. The four stars, HS 2324+3944, NGC 7094, Abell 43, and Sh 2-68, define a small group of peculiar PG 1159 stars (termed hybrid PG 1159). Unlike all other PG 1159 stars hydrogen is detected in their spectra. Three of them are CSPNe. Our Non-LTE analyses (Dreizler et al 1995; Dreizler et al 1996) show that these stars have typical PG 1159 Teff, log g as well as C and He abundances (Table 1). In contrast, the O abundance is lower than in PG 1159 stars. N is probably present but near the detection limit of the currently available spectra. Hybrid PG 1159 stars tend to have lower masses/luminosities than ordinary PG 1159 stars. A reduced mass-loss in their post-AGB evolution might be responsible for the incomplete removal of the H rich envelope. However, peeling of a post-AGB star alone can not produce the observed abundance pattern. In addition, mixing is required. A first evolution calculation with time dependent mixing of Iben & MacDonald (1995) shows some qualitative agreement in the abundance pattern like the C/He ratio it is, however, not able to explain the overall abundances.

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Copyright

References

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Ciardullo, R., Bond, H.E. 1996, AJ 111, 2332.
Dreizler, S., Werner, K., Heber, U. 1995, in Lecture Notes in Physics 443, p. 160.
Dreizler, S., Werner, K., Heber, U., Engels, 1996, A&A 309, 820.
Iben, I. Jr., MacDonald, J. 1995, in Lecture Notes in Physics 443, 48.
Silvotti, R. 1996, A&A 309, L23.

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