Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Blöcker:
Evolution on the AGB and beyond:
On the formation of H-deficient post-AGB stars
in
Low-mass Wolf-Rayet stars: Origin and
evolution
Astrophysics and Space Science 275, 1-14 (2001)
Abstract.
The evolution on the AGB and beyond is reviewed with respect to the
origin of Wolf-Rayet central stars. We focus on thermal pulses due to
their particular importance for the evolution of hydrogen deficient
stars. It is shown that overshoot applied to all convection regions is
a key ingredient to model these objects leading to
intershell abundances already close to the surface abundances of
Wolf-Rayet
central stars. In contrast to standard evolutionary calculations,
overshoot models do show dredge up for very low envelope masses and
efficient dredge up was found even during the post-AGB stage. Three
thermal pulse scenarios for Wolf-Rayet central stars can now
be distinguished: an AGB Final Thermal Pulse (AFTP) occurring at the
very end of the AGB evolution, a Late Thermal Pulse (LTP) occurring
during the post-AGB
evolution when hydrogen burning is still on, and a Very Late Thermal
Pulse (VLTP) occurring on the cooling branch when hydrogen burning has
already ceased. All scenarios lead to hydrogen-deficient post-AGB stars
with carbon and oxygen abundances as observed for Wolf-Rayet stars.
Hydrogen is either diluted by dredge up (AFTP, LTP) or completely burnt
(VLTP).
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