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 with low-mass (≲2 M_⊙) and intermediate-mass (∼2 M_⊙-M_BH) donors leads to severe constraints on the evolutionary parameters of the progenitors. In particular, we find that (1) BH progenitor masses cannot exceed about 2M_BH; (2) high values of the common envelope efficiency parameter (α_CE > 1) are required, implying that energy sources other than orbital contraction must be invoked to eject the envelope; and (3) the mass-loss fraction in helium star winds is limited to ≲50%. Outside of this limited parameter space for progenitors we find that either BH X-ray binary formation cannot occur at all or donors do not have the full range of observed masses. We discuss the implications of these results for the structure of massive hydrogen-rich stars, the evolution of helium stars, and BH formation. We also consider the possible importance of asymmetric kicks.