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

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Letter to the Editor

On the formation of hydrogen-deficient post-AGB stars

F. Herwig 1, T. Blöcker 2, N. Langer 1 and T. Driebe 2

1 Universität Potsdam, Institut für Physik, Astrophysik, Am Neuen Palais 10, D-14469 Potsdam, Germany (fherwig, ntl@astro.physik.uni-potsdam.de)
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany (bloecker, driebe@speckle.mpifr-bonn.mpg.de)

Received 19 July 1999 / Accepted 9 August 1999


We present an evolutionary sequence of a low mass star from the Asymptotic Giant Branch (AGB) through its post-AGB stage, during which its surface chemical composition changes from hydrogen-rich to strongly hydrogen-deficient as consequence of a very late thermal pulse, following the so-called born-again scenario . The internal structure and abundance changes during this pulse are computed with a numerical method which allows the physically consistent calculation of stellar layers where thermonuclear and mixing time scale are comparable - a situation which occurs when the helium flash driven convection zone extends to the hydrogen-rich surface layers during the pulse peak. The final surface mass fractions are [He/C/O]=[0.38/0.36/0.22], where the high oxygen abundance is due to diffusive overshoot employed during the AGB evolution. These models are the first to achieve general agreement with the surface abundance pattern observed in hydrogen-deficient post-AGB stars - e.g. the PG 1159 stars or the WR-type central stars of planetary nebulae -, confirming the born-again scenario with a physically consistent calculation and supporting the occurrence of convective overshooting in thermally pulsing AGB stars.

Key words: stars: abundances – stars: AGB and post-AGB – stars: evolution – stars: interiors

Send offprint requests to: F. Herwig

This article contains no SIMBAD objects.


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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999