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Astron. Astrophys. 349, L5-L8 (1999)

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1. Introduction

Stars on the so called Asymptotic Giant Branch (AGB) have strong stellar winds, which gradually reduce the mass of the hydrogen-rich stellar envelope. When this envelope mass falls below a critical value, the stars leave the AGB to become post-AGB stars, central stars of planetary nebulae (CSPNe), and finally white dwarfs. Post-AGB stars show a variety of surface abundances (Mendez, 1991). About [FORMULA] of all CSPNe show a solar-like composition while the remaining ones are hydrogen-deficient. Among the latter are Wolf-Rayet type CSPNe ([WR]-CSPNe) and the extremly hot PG 1159 stars with typical surface abundances of [He/C/O]=[0.33/0.50/0.17] (Dreizler and Heber (1998); see also Koesterke and Hamann (1997) and references in both papers). Hydrogen-deficiency is also found in white dwarfs of spectral type DO (Dreizler and Werner, 1996).

The origin of the hydrogen-deficiency in post-AGB stars is a longstanding problem. Most post-AGB calculations predict a hydrogen-rich surface composition (Schönberner, 1979; Schönberner, 1983; Wood and Faulkner, 1986; Vassiliadis and Wood, 1994; Blöcker, 1995a; Blöcker and Schönberner, 1997). So far, no post-AGB models reproduced the observed high carbon and oxygen abundance. The most promising scenario for obtaining a hydrogen-deficient surface composition envokes a very late thermal pulse (Fujimoto, 1977; Schönberner, 1979; Iben et al., 1983) - i.e. a pulse which occurs after the star has already left the AGB - during which the pulse driven convection zone can mix hydrogen-free material out to the stellar surface. Within this born-again scenario , Iben and McDonald (1995) obtain surface mass fractions of [He/C/O]=[0.76/0.15/0.01], i.e. their model indeed became strongly hydrogen-deficient. Hovever, the large oxygen abundance found in most H-deficient post-AGB stars could not be reproduced be these, nor by any other calculation. These difficulties have posed a strong limitation to the whole scenario.

In this Letter we present a post-AGB model sequence starting from an AGB model computed with overshoot (Herwig et al., 1997), and using a numerical method of computing nuclear burning and time-dependent convective mixing simultaneously.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999