Donor Stars in Black Hole X‐Ray Binaries

We study theoretically the formation of black-hole (BH) X-ray binaries.Consistency of the models with the observed relative numbers of systems withlow-mass (<2 M_sun) and intermediate-mass (~2 M_sun - M_{BH}) donors leads tosevere constraints on the evolutionary parameters of the progenitors. Inparticular, we find that (i) BH progenitor masses cannot exceed about 2 M_{BH};(ii) high values of the common-envelope efficiency parameter (alpha_{CE} > 1)are required, implying that energy sources other than orbital contraction mustbe invoked to eject the envelope; (iii) the mass-loss fraction in helium-starwinds is limited to be <50%. Outside of this limited parameter space forprogenitors we find that either BH X-ray binary formation cannot occur at allor donors do not have the full range of observed masses. We discuss theimplications of these results for the structure of massive hydrogen-rich stars,the evolution of helium-stars, and BH formation. We also consider the possibleimportance of asymmetric kicks.Comment: 29 pages, 6 figures, accepted for publication in The Astrophysical Journa