The Limiting Stellar Initial Mass for Black Hole Formation in Close Binary Systems

Abstract

We present models for the complete life and death of a 60 solar mass star evolving in a close binary system, from the main sequence phase to the formation of a compact remnant and fallback of supernova debris. After core hydrogen exhaustion, the star expands, loses most of its envelope by Roche lobe overflow, and becomes a Wolf-Rayet star. We study its post-mass transfer evolution as a function of the Wolf-Rayet wind mass loss rate (which is currently not well constrained and will probably vary with initial metallicity of the star). Varying this mass loss rate by a factor 6 leads to stellar masses at collapse that range from 3.1 to 10.7 solar masses. Although the iron core masses at collapse are generally larger for stars with larger final masses, they do not depend monotonically on the final stellar mass or even the C/O-core mass. We then compute the evolution of all models through collapse and bounce. The results range from strong supernova explosions for the lower final masses to the direct collapse of the star into a black hole for the largest final mass. Correspondingly, the final remnant masses, which were computed by following the supernova evolution and fallback of material for a time scale of about one year, are between 1.2 and 10 solar masses. We discuss the remaining uncertainties of this result and outline the consequences of our results for the understanding of the progenitor evolution of X-ray binaries and gamma-ray burst models.