The evolutionary time scale of Sakurai's object: A test of convection theory?
Abstract
Sakurai's object (V4334 Sgr) is a born again AGB star following a very late thermal pulse. So far no stellar evolution models have been able to explain the extremely fast evolution of this star, which has taken it from the pre-white dwarf stage to its current appearance as a giant within only a few years. A very high stellar mass can be ruled out as the cause of the fast evolution. Instead the evolution time scale has now been found in stellar models by making the assumption that the convective efficiency for element mixing in the He-flash convection zone during the very late thermal pulse is smaller 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 can be accelerated to a few years, as required by past observations of V4334 Sgr. Assuming a mass of V4334 Sgr of 0.604M_sun which is consistent with a distance of 4kpc, a reduction of the MLT mixing efficiency by a factor of ~100 is required to match the evolutionary time scale of V4334 Sgr. This value decreases if V4334 Sgr has a smaller mass and accordingly a smaller distance. However, the effect does not disappear for the smallest possible masses. These findings may present a semi-empirical constraint on the element mixing in convective zones of the stellar interior.Keywords
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