Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Driebe and T. Blöcker:
Stellar Evolution with Rotation: Mixing
Processes in AGB Stars
Astronomische Gesellschaft Meeting Abstracts (AGM 17,
P16), Dynamic
Instabilities and Stability in the Universe
Annual Scientific Meeting of the Astronomische Gesellschaft, Sep 18-23,
2000, Bremen, Germany
Abstract.
We included diffusive angular momentum transport and rotationally
induced
mixing processes in our stellar evolution code and studied the
influence of
rotation on the evolution of intermediate mass stars
(M=2...6 Msun) towards and along the asymptotic giant branch (AGB).
The calculations start in the fully convective pre-main sequence phase
and the initial angular momentu m was adjusted such
that on the zero-age main sequence vrot=200 km/ s is achieved. The
diffusion
coefficients for the five rotational instabilities considered
(dynamical shear, secular shear, Eddington-Sweet (ES) circulation,
Solberg-Hoiland-instability and
Goldreich-Schubert-Fricke (GSF) instability) were adopted from
Heger et al. (2000, ApJ 528, 368). Mixing efficiency and sensitivity of
these
processes against molecular weight gradients have been determined by
calibration of the main sequence width. In
this study we focus on the abundance evolution of carbon. On the one
hand, the surface abundance ratios of 12C/13C a nd 12C/16O at the base
of the AGB were found to be 7...10 and 0.1, resp., being a factor of
two lower than in
non-rotating models. This results from the slow but continuously
operating
rotationally induced mixing due to the ES-circulation and the
GSF-instability
during the long main sequence phase. On the other hand, 13C serves as
neutron
source for interior
s-process nucleosynthesis in AGB stars vi a 13C(alpha,n)16O. Herwig et
al.
(1997, A&A 324, L81) found that a 13C pocket is forme d in the
intershell
region of 3 M⊙ AGB star if diffusive overshoot is considered.
Our calculations show, that mixing
processes due to rotation open an alternative channel for the formation
of a 13C pocket as found by Langer et al. (1999, A&A 346, L37).
Again, ES-circulation and GSF-instability are the predominant
rotational mixing processes.