Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
F. Herwig, T. Blöcker, N. Langer and T.
Driebe:
On the formation of hydrogen-deficient
post-AGB stars
Astronomy and Astrophysics 349, L5-L8 (1999)
Abstract.
We present an evolutionary sequence of a low mass star from the
Asymptotic Giant Branch (AGB) through its post-AGB stage, during which
its
surface chemical composition changes from hydrogen-rich to strongly
hydrogen-deficient as consequence of a very late thermal pulse,
following the
so-called born-again scenario. The internal structure and abundance
changes
during this pulse are computed with a numerical method which allows the
physically consistent calculation of stellar layers where thermonuclear
and
mixing time scale are comparable - a situation which occurs when the
helium
flash driven convection zone extends to the hydrogen-rich surface
layers during
the pulse peak. The final surface mass fractions are
[He/C/O]=[0.38/0.36/0.22],
where the high oxygen abundance is due to diffusive overshoot employed
during
the AGB evolution. These models are the first to achieve general
agreement with the surface abundance pattern observed in
hydrogen-deficient post-AGB
stars - e.g. the PG 1159 stars or the WR-type central stars of
planetary
nebulae -, confirming the born-again scenario with a physically
consistent
calculation and supporting the occurrence of convective overshooting in
thermally pulsing AGB stars.
You can get this publication ...