T. Rauch(1), S. Dreizler(2), and K. Werner(1)
(1)Institut für Astronomie und Astrophysik, Astronomie, Universität
Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2)Institut für Astronomie und Astrophysik, Universität Kiel, D-24098 Kiel, Germany
Contribution to the 10th European Workshop - White Dwarfs, Blanes, Spain, June 17-21, 1996
Abstract. The very hot, H-deficient PG 1159 stars are found in the post-AGB region of the Hertzsprung-Russell diagram. Most likely the 'born-again' post-AGB scenario is valid for their evolution. It predicts a final He-flash which brings back the star to the AGB, followed by a second (He burning) descent from the AGB where a superwind may take off the entire H-rich and most of the He-rich envelope and lay bare intershell layers, where Ne is the fourth abundant element after He, C, and O. In order to support this hypothesis, we present a search for the Ne VII 3644 Å absorption line in several of these stars.
The two presently known O(He) central stars of planetary nebulae, K 1-27 and LoTr 4, exhibit almost pure He II absorption line spectra. They are found in the post-AGB region of the Hertzsprung-Russell diagram just amongst the PG 1159 stars which in contrast show additional strong C IV lines. We present an analysis of three recently discovered O(He) stars, HS 0742+6520, HS 1522+6615 and HS 2209+8229, for which no associated planetary nebulae have been detected. It is likely that there exists an evolutionary channel for H-deficient post-AGB stars, 'O(He) stars -> DO white dwarfs', which is separate from to the well established 'Wolf-Rayet [WC] central stars -> PG 1159 stars -> DO white dwarfs' evolutionary sequence.
Paper (123k gzip'ed Postscript including figures)
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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
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