Helium-Star Mass Loss and its Implications for Black-Hole Formation and Supernova Progenitors

Recently the observationally derived stellar-wind mass-loss rates for Wolf-Rayet stars, or massive naked helium stars, have been revised downwards by a substantial amount. We present evolutionary calculations of helium stars incorporating such revised mass-loss rates, as well as mass transfer to a close compact binary companion. Our models reach final masses well in excess of 10 Msun, consistent with the observed masses of black holes in X-ray binaries. This resolves the discrepancy found with previously assumed high mass-loss rates between the final masses of stars which spend most of their helium-burning lifetime as Wolf-Rayet stars (about 3 Msun) and the minimum observed black hole masses (6 Msun). Our calculations also suggest that there are two distinct classes of progenitors for Type Ic supernovae: one with very large initial masses (> 35 Msun), which are still massive when they explode and leave black hole remnants, and one with moderate initial masses (about 12 - 20 Msun) undergoing binary interaction, which end up with small pre-explosion masses and leave neutron star remnants.