Triple-alphs burning products on the surface of peculiar post-AGB stars

K. Werner1, S.Dreizler2, U.Heber2, T.Rauch1

1Institut für Theoretische Physik und Sternwarte der Universität Kiel, Germany
2Sternwarte Bamberg, Universität Erlangen-Nürnberg, Germany

The so-called PG 1159 stars form a new spectroscopic class of extremely hot hydrogen-deficient post-AGB stars. Our spectral analyses with model atmospheres show that their chemical surface composition is dominated by carbon, helium, and oxygen. We suggest that these peculiar stars have suffered a late helium-shell flash which has caused the removal of the hydrogen-rich envelope and even most of the helium-rich intershell matter. The idea that the former helium-burning region is now exposed at the surface of the PG 1159 stars is supported by the recent detection of a very high amount of neon in some objects.

The most extreme object is H 1504+65 which is one of the brightest X-ray sources in the sky and the hottest star ever analyzed with model atmosphere techniques ( Teff=170000 K). The photosphere is devoid of hydrogen and helium and composed of oxygen and carbon by equal amounts! This means that we look at the naked core of the former Red Giant. This gives us the unique possibility to confine empirically the still uncertainly known 12C( alpha , gamma) 16O nuclear reaction rate.

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