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Institut für Astronomie und Astrophysik

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Preprint 09n/10

HST/COS Observations of O(He) Stars

T. Rauch and K. Werner

Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics,
Eberhard Karls University, D-72076 Tübingen, Germany

To be published in: Proceedings of the 17th European White Dwarf Workshop, AIP Conference Series

Abstract. The four known O(He) stars are the only amongst the hottest post-AGB stars (Teff > 100,000 K) whose atmospheres are composed of almost pure helium. This chemistry markedly differs from that of the hydrogen-deficient post-AGB evolutionary sequence with objects that have carbon-dominated atmospheres (namely PG1159 stars and Wolf-Rayet type central stars). While PG1159 and Wolf-Rayet stars are the result of a (very) late helium-shell flash, this scenario cannot explain the O(He) stars. Instead, they are possibly double-degeneratemergers.We speculate that the four O(He) stars represent evolved RCrB stars, which also have helium-dominated atmospheres.
We present UV spectroscopy of the O(He) stars based on recently performed HST/COS observations.

Preprint (376 kb PDF file including figures)

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