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Preprint 12e/08

The Evolution of O(He) Stars

T. Rauch (1), K. Werner (1), J. W. Kruk (2)

(1) Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Tübingen, Germany
(2) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, U.S.A.

To be published in: Proceedings: Future Directions in Ultraviolet Spectroscopy Conference, Annapolis, MD, 2008

Abstract. Spectral analyses of H-deficient post-AGB stars have shown that a small group of four extremely hot objects exists which have almost pure He absorption-line spectra in the optical. These are classified as O(He) stars. For their origin there are two scenarios: They could be the long-sought hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double-degenerate merging process. An alternative explanation might be that O(He) stars are post early-AGB stars. These depart from the AGB just before they experience their first thermal pulse (TP) which will then occur as a late thermal pulse (LTP). This would be a link to the low-mass He-enriched sdO stars and low-mass, particularly He-rich PG 1159 stars. We present results of a spectral analysis of FUSE observations of the O(He) stars by means of NLTE model-atmosphere techniques and summarize our p resent picture of their evolutionary scenario.
 
Key words: Stellar evolution, Stellar atmospheres, Mass loss, Abundances, white dwarfs

Preprint (38 kb PDF file including figures)

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