The Evolutionary Timescale of Sakurai’s Object: A Test of Convection Theory?

Sakurai's object (V4334 Sgr) is a born again AGB star following a very latethermal pulse. So far no stellar evolution models have been able to explain theextremely fast evolution of this star, which has taken it from the pre-whitedwarf stage to its current appearance as a giant within only a few years. Avery high stellar mass can be ruled out as the cause of the fast evolution.Instead the evolution time scale is reproduced in stellar models by making theassumption that the efficiency for element mixing in the He-flash convectionzone during the very late thermal pulse is smaller than predicted by themixing-length theory. As a result the main energy generation from fast protoncapture occurs closer to the surface and the expansion to the giant state isaccelerated to a few years. Assuming a mass of V4334 Sgr of 0.604Msun -- whichis consistent with a distance of 4kpc -- a reduction of the mixing lengththeory mixing efficiency by a factor of ~ 100 is required to match itsevolutionary time scale. This value decreases if V4334 Sgr has a smaller massand accordingly a smaller distance. However, the effect does not disappear forthe smallest possible masses. These findings may present a semi-empiricalconstraint on the element mixing in convective zones of the stellar interior.Comment: 16 pages, 3 figures, ApJ Letter, in press; some additional information as well as modifications as a result of the refereeing process, improved layout of prev. Fig.1 (now Fig.1 and Fig.2