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Preprint 06/00

PG 1159 stars

Klaus Werner

Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

To appear in: Encyclopedia of Astronomy and Astrophysics, IOP Publishing

Abstract. The PG 1159 stars form a small group of about 30 objects. They are named after the prototype PG 1159-035 ( = GW Vir), which has been discovered in the Palomar-Green Survey. They are hot post-Asymptotic Giant Branch (post-AGB) stars and their location in the H-R diagram coincides with the hottest central stars of planetary nebulae (CSPN) and hottest white dwarfs (WD). The most peculiar characteristic is their chemical surface composition. They are hydrogen-deficient and the most abundant elements are He, C, and O with typical abundances of 33%, 50%, 17%, respectively. This suggests that their immediate progenitors are CSPN with Wolf-Rayet type spectra, whose analyses yield similar abundance patterns. It is thought that most of the PG 1159 stars evolve into non-DA WDs, i.e. helium-rich objects from which the heavy elements were removed from the surface layer by gravitational settling. The origin of the PG 1159 surface chemistry is under debate. The most popular picture describes these stars as the result of a late He-shell flash, i.e. the re-ignition of helium fusion in a CSPN or even in a cooling, contracting WD. This event causes the star to return onto the AGB and to retrace its post-AGB evolution for a second time (born-again AGB star scenario). As a consequence, envelope convection causes ingestion and burning of surface hydrogen and, possibly in conjunction with mass-loss, carbon and oxygen as helium burning products are mixed into surface layers.

The mere existence of PG 1159 stars represents a challenge to post-AGB stellar evolution theory. Details of the born-again scenario are being worked out, in order to prove or disprove it. An alternative scenario proposes that close binary evolution causes stripping of the envelope during the red giant phase. Since some PG 1159 stars are associated with a PN, it is hoped that analyses of the PNe give further hints as to the stellar history. Some PG 1159 stars are variable and these multi-periodic g-mode pulsators are defining the GW Vir (or DOV) instability strip in the HR-diagram. The pulsational properties are used to probe the interior structure of these stars. Hence spectroscopic analyses with model atmospheres in combination with asteroseismology using pulsating stellar models help to constrain theoretical evolutionary calculations for PG 1159 stars.

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Paper (96k gzip'ed Postscript including figures)

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