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

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1. Introduction

Towards the end of their lifetime, stars of low and intermediate mass ([FORMULA]) evolve along the Asymptotic Giant Branch (AGB) stage Iben & Renzini 1983; Habing 1996; Lattanzio & Boothroyd 1997. The core of carbon and oxygen is surrounded by a sandwich like structure consisting of a helium burning shell, the hydrogen burning shell and the intershell region in between. Evolved AGB stars undergo recurrent thermal instabilities of the helium burning shell (He-flash) which are referred to as thermal pulses (TP) Schwarzschild & Härm 1965; Weigert 1966. Locally, helium burning peak luminosities of [FORMULA] are released and cause complex convective mixing events. The He-flash causes a pulse-driven convective zone (PDCZ) in the intershell region. After the He-flash the bottom boundary of the convective envelope may engulf the deeper regions where material previously synthesized by hydrogen and helium burning is present (third dredge-up, TDUP).

Despite many studies of AGB evolution Iben 1976; Schönberner 1979; Lattanzio 1986; Boothroyd & Sackmann 1988a; Vassiliadis & Wood 1993; Blöcker 1995; D'Antona & Mazzitelli 1996; Forestini & Charbonnel 1997; Straniero et al. 1997; Wagenhuber & Groenewegen 1998, important details, like the surface enrichment with nuclear processed material from the deep interior (dredge-up) or the origin of [FORMULA] which is an important source of neutrons for the synthesis of heavy elements in AGB stars, are not very well understood. To improve this situation we present the structural properties as well as the chemical evolution of the interior of AGB stellar models with convective overshoot. Convective motions of matter approach the convective boundary with a non-zero velocity and penetrate into the formally stable region. These overshooting flows lead to extra mixing of elements. Canuto Canuto (1998) has pointed out that overshoot is a dynamical consequence of Newton's laws and as such is unavoidable. Previous studies have shown that models with overshoot can account for several observed properties of AGB and post-AGB stars (Blöcker et al. 1997; Herwig et al. 1997, 1998, 1999b).

We give a brief review of AGB star properties relevant for our new models (Sect. 2), some remarks on the stellar evolution code as well as some information on overshoot and its treatment in our models (Sect. 3). We explain the differences between models without overshoot and with overshoot by looking separately at the two relevant convective boundaries: the bottom of the envelope convection (Sect. 4) and the bottom of the PDCZ (Sect. 5). The chemical and structural surface properties of models with overshoot are described in Sect. 6. Conclusions are presented in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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