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

Abstract

Sakurai's Object (V4334 Sagittarii) is a born-again asymptotic giant branch 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 timescale is reproduced in stellar models by making the assumption that the 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 (MLT). As a result, the main energy generation from fast proton capture occurs closer to the surface, and the expansion to the giant state is accelerated to a few years. Assuming a mass of V4334 Sgr of 0.604 M☉—which is consistent with a distance of 4 kpc—a reduction of the MLT mixing efficiency by a factor of ~100 is required to match its evolutionary timescale. 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 semiempirical constraint on the element mixing in convective zones of the stellar interior.