The evolution of AGB stars with convective overshoot
F. Herwig
The influence of
extended convective mixing (overshoot) on
asymptotic giant branch stars is
investigated in detail. The extended mixing
is treated time-dependently, and the efficiency declines
exponentially with the geometric distance from the convective
boundary. It has been considered at all convective boundaries,
including the He-flash convection zone in the intershell
region which forms during the thermal pulses.
Both the structural and the chemical evolution are affected
by the inclusion of overshoot. The main results include a very
efficient third dredge-up which leads to the formation
of carbon stars of low mass and luminosity. A 13C pocket which
may serve as a neutron source for the s-process can
form after the third
dredge-up has reached into the 12C rich intershell.
Overshoot applied to the pulse-driven convective
zone during the He-flash
leads to a deeper penetration of the bottom of this
convective zone into the C/O core below the He-burning
shell. This in turn
causes 4He to be less abundant in the intershell while
12C and 16O are strongly enriched in comparison to calculations
without overshoot. We show that overshoot at the He-flash
convection zone and the dredge-up efficiency are closely
related. Characteristic properties of the structural and
chemical evolution of AGB stars are presented.
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