Modeling the evolution of Sakurai's object

F. Herwig

Universität Potsdam, Institut für Physik, Astrophysik
Institute for Physics and Astronomy, University of Victoria, Canada

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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