Modeling the evolution of Sakurai's Object

Abstract

Sakurai's Object is a born again AGBstar of the very late thermal pulse flavor. In thiscontribution I will discuss new models of stellar evolution andnucleosynthesis models of this phase. Two most intriguing properties ofSakurai's Object have so far not been understood theoretically: the peculiar chemical appearance, in particular the high lithiumabundance, and the short time scale of only a few years on which thetransition from the dwarf configuration into the born again giantappearance has occurred. A new nucleosynthesis mode of hot hydrogen-deficient ³He burning can explain the extraordinarylithium abundance. During the thermal pulse ³He is ingested fromthe envelope together with the protons into the hot He-flashconvection zone. The first network calculations show that, due to thelarge ¹²C abundance protons are captured rather by carbon, thandestroy newly formed ⁷Be and ultimately ⁷Li. Moreover, the shortevolution time scale has been reproduced by making the assumption that the convective efficiency forelement mixing is smaller by two to three orders of magnitude thanpredicted by the mixing-length theory. As a result the main energygeneration from fast convective proton capture will occur at a largermass coordinate, closer to the surface and the expansion to the giantstate is accelerated to a few years, in excellent agreement with thebehavior of Sakurai's Object. This result represents an independent empiricalconstraint on the poorly known efficiency of element mixing inconvective zones of the stellar interior.