Astron. Astrophys. 360, 952-968 (2000)
1. Introduction
Towards the end of their lifetime, stars of low and intermediate
mass ( ) 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
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 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.
© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000
helpdesk.link@springer.de  |