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Astron. Astrophys. 321, 485-491 (1997)

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The influence of gravitational settling and selective radiative forces in PG 1159 stars. II

K. Unglaub and I. Bues

Dr. Remeis-Sternwarte Bamberg, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstraße 7, D-96049 Bamberg, Germany

Received 22 April 1996 / Accepted 17 July 1996

Abstract

Previous investigations have shown that time-independent diffusion calculations cannot explain the large abundances of carbon and oxygen in the helium-dominated atmospheres of hot pre-white dwarfs. Therefore we investigate in this paper the time evolution of the chemical composition in the outer regions with surface layer masses [FORMULA]. The elements He, C, N, O and Ne are taken into account. It is shown that for [FORMULA] and [FORMULA] and [FORMULA], respectively, gravitational settling should lead to a strong depletion of both elements within a time-scale of 1000 y. According to theoretical evolutionary tracks for post-AGB stars, PG 1159 stars with [FORMULA] should have passed through regions in the HRD with [FORMULA] and [FORMULA]. For these model parameters the radiative forces are strong enough to transform a helium-rich atmosphere with only traces of heavy elements into a metal-rich one. However, this transformation takes about 800000 y, long compared to the corresponding time-scales of stellar evolution. For more massive objects, the observational results cannot be explained by diffusion processes alone.

Key words: stars: abundances – stars: evolution – diffusion – white dwarfs

Send offprint requests to: K. Unglaub

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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