Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
F. Herwig, T. Blöcker and D.
Schönberner:
The Role of Convective Boundaries
Abstract (contributed talk) for
IAU Symp. 191, Asymptotic Giant Branch Stars, Aug 27th - Sep 1rst,
1998,
Montpellier, France
Abstract.
We have investigated the influence of convective overshoot
on stellar evolution models of the thermal pulse AGB phase.
Our model sequences
from the main sequence to the AGB for different
intermediate masses include about a dozen thermal pulses
respectively. An exponential diffusive overshoot
algorithm (Herwig et al. 1997)
based on radiation hydrodynamics simulations (Freytag et al. 1996)
has been applied to all convective boundaries during all evolutionary
stages.
We demonstrate that this treatment has important
consequences for the complex mixing events and the nuclear processing
of elements associated with the occurrence of thermal pulses on the
AGB (Herwig 1998). The models show that the third dredge-up is
dependent on time
(thermal pulse number) and initial core mass at the first thermal
pulse.
For instance, in the case of the 3Msol model sequence
the onset of dredge up takes place at the
third thermal pulse. Over the following
pulses the dredge-up efficiency increases steadily until
the core mass reduction after the pulse exceeds the core growth during
he interpulse due to hydrogen shell burning (lambda > 1).
These models satisfy the observational constraint that
carbon stars have lower core masses (0.6 Msol)
than usually found by most stellar evolution models.
Another important consequence of the diffusive overshoot treatment is
the establishment of a so-called C13-pocket at the
hydrogen-carbon interface, which emerges when
the convective envelope bottom has reached its deepest position
after the thermal pulse in the case of efficient dredge-up.
The formation of this C13 enrichment is a direct result of the partial
mixing of protons and C12 which follows from the
diffusive overshoot description.
References:
Freytag B., Ludwig H.-G., Steffen M. 1996, A&A 313, 497
Herwig, F., 1998, PhD Thesis, Universität Kiel
Herwig, F., Blöcker, T., Schönberner, D., El Eid, M.,
1997, A&A 324, L81
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