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Overshoot in Giant Stars

By F. Herwig and T. Blöcker


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 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.

Year: 1999
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