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Astron. Astrophys. 324, L81-L84 (1997)

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Letter to the Editor

Stellar evolution of low and intermediate-mass stars

IV. Hydrodynamically-based overshoot and nucleosynthesis in AGB stars

F. Herwig 1, T. Blöcker 2, D. Schönberner 1 and M. El Eid 3

1 Astrophysikalisches Institut Potsdam, D-14773 Potsdam, Germany (fherwig@aip.de; deschoenberner@aip.de)
2 Institut für Astronomie und Astrophysik, Universität Kiel, D-24098 Kiel, Germany (bloecker@astrophysik.uni-kiel.de)
3 American University of Beirut, Department of Physics, Beirut, Lebanon (meid@aub.edu.lb)

Received 13 March 1997 / Accepted 20 May 1997

Abstract

The focus of this study is on the treatment of those stellar regions immediately adjacent to convective zones. The results of hydrodynamical simulations by Freytag et al. (1996) show that the motion of convective elements extends well beyond the boundary of the convectively unstable region. We have applied their parametrized description of the corresponding velocities to the treatment of overshoot in stellar evolution calculations up to the AGB (Pop. I, [FORMULA]).

Our calculations show the [FORMULA] dredge-up already at the [FORMULA] thermal pulse (TP), and the dredge-up parameter reaches [FORMULA] during the next five pulses. Accordingly, the amount of dredged up 12 C is up to [FORMULA]. Our models develope a small so-called 13 C-pocket consisting of a few [FORMULA]. Finally, this treatment of boundaries of convective regions leads to intershell abundances of typically (4 He/12 C/16 O)=(23/50/25) (compared to (70/26/1) in the standard treatment).

Key words: Stars: evolution – Stars: AGB, post-AGB – Stars: abundances

Send offprint requests to: F. Herwig

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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