Empirical Lower Mass Limit for Black-Hole Formation in a Massive Binary

Abstract

Observations of massive binaries provide importantconstraints on the evolutionary fate of massive stars. Although stellarevolution in a binary system is different from single-star evolution,practically the only way to derive information on the progenitor masses ofneutron stars and black holes is by studying massive binaries andtheir descendants. Wolf-Rayet binaries show a rather continuous massdistribution, while there might be a gap between the neutron-starand black-hole masses. Wray 977 (GX301-2) hosts the most massiveOB star with X-ray pulsar companion, and sets an empirical lowermass limit for black-hole formation in a massive binary. HD153919 isthe OB supergiant with earliest spectral type, and potentially themost massive OB star in a high-mass X-ray binary, but the nature ofthe X-ray source (4U1700-37) is not clear. Although it might be alow-mass black hole, new observations suggest it is a neutron star.