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F. Herwig and T. Blöcker:
Overshoot in Giant Stars
Proceedings of the 35th Liège International Astrophysics Colloquium,
The Galactic Halo: From Globular Clusters to Field Stars,
A. Noels, P. Magain, D. Caro, E. Jehin, G. Parmentier and A. Thoul
(eds.),
Universite de Liège, Belgium, in press (1999)
Abstract.
The concept of overshoot has already been considered for numerous cases
in stellar evolution calculations. We explore the consequences of
overshoot at the convection zone which forms during the He-flash
(thermal pulse) in AGB stars. We find dramatic changes for the
abundances within the intershell region as well as for the mechanism of
the 3$^\mathrm{rd}$ dredge-up. That means that both the predicted
evolution of structural quantities as well as the chemical evolution at
the surface will be different, if overshoot is considered. We also
present evidence for the presence of overshoot during the He-flash from
detailed model calculations of the post-AGB phase and the comparison
with observations. The new evolutionary models show that a very late
thermal pulse during the post-AGB evolution can bring the intershell
material up to the surface with minimal modification due to convective
H-burning. The good agreement of the surface abundances of these models
with observed surface abundances of H-deficient post-AGB stars is
interpreted as a strong support for the presence of overshoot during
the thermal pulses of AGB stars.
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