Modeling the evolution of Sakurai's object
F. Herwig
Sakurai's object is a born again AGB
star of the very late thermal pulse flavor. In this
contribution I will discuss new models of stellar evolution and
nucleosynthesis models of this phase. Two most intriguing properties of
Sakurai's objects have so far not been understood theoretically: the
peculiar chemical appearance, in particular the high lithium
abundance and the short time scale of only a few years on which the
transition from the dwarf configuration into the born again giant
appearance has occurred. A new nucleosynthesis mode of \emph{hot
hydrogen-deficient 3He burning can explain the extraordinary
lithium abundance. During the thermal pulse 3He is ingested from
the envelope together with the protons into the hot He-flash
convection zone. The first network calculations show that due to the
large 12C abundance protons are rather captured by carbon than
destroy newly formed 7Be and ultimately 7Li. Moreover, the short
evolution time scale has been
reproduced by making the assumption that the convective efficiency for
element mixing is smaller by two to three orders of magnitude than
predicted by the mixing-length theory. As a result the main energy
generation from fast convective proton capture will occur at a larger
mass coordinate, closer to the surface and the expansion to the giant
state is accelerated to a few years in excellent agreement with
Sakurai's behavior. This result represents an independent empirical
constraint on the poorly known efficiency of element mixing in
convective zones of the stellar interior.
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