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Astron. Astrophys. 360, 952-968 (2000)

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7. Conclusions

We have studied how the evolution of AGB stars is affected by exponential diffusive overshoot at all convective boundaries. The main differences between models with overshoot compared to models without overshoot are

    • a depletion of helium and enhancement of carbon and oxygen in the intershell abundance distribution which is caused by,

    • a deeper penetration of the He-flash convection zone into the C/O core below (intershell or fourth dredge-up),

    • larger energy generation by He-burning during the thermal pulse and consequently,

    • higher temperature at the bottom of the He-flash convection zone and lower temperature at the top,

    • substantially increased dredge-up of material which has a modified abundance distribution compared to models without overshoot and as a result,

    • a different evolution of the surface abundances showing a stronger signature of nucleosynthesis and

    • a constant or decreasing core mass during some more advanced TPs,

    • a faster recovery of the hydrogen-burning shell after the TP and correlated with this,

    • a faster recovery of the surface abundance after the TP compared to models without overshoot, and

  1. the formation of a [FORMULA] and a [FORMULA] pocket at the envelope-core interface at the end of the dredge-up.

These properties of AGB evolution models improve the understanding of observationally obtained information about AGB stars. To date no obvious contradictions of observational properties and AGB models with overshoot have appeared. On the other hand, overshoot in AGB stellar models (in particular that at the base of the He-flash convection zone) has been an indispensable precondition for the starting point of H-deficient post-AGB model sequences which can explain the observed abundance ratios of these stars, in particular their oxygen abundance Herwig et al. 1999a. Therefore, overshoot should be considered an important ingredient for the modeling of AGB stars.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000