Formation of High Mass X-ray Black Hole Binaries

We discuss the progenitor evolution of high mass black hole binaries with black hole of mass ∼ > 10M⊙, like Cyg X-1. From the observed large space velocity of Cyg X-1, which we interpret as coming from a Blaauw-Boersma kick, we find that the helium envelope of the black hole progenitor must have been at least 16M⊙. Using new and improved wind mass loss rates for Wolf-Rayet stars, we are unable to evolve such massive helium envelopes unless the black hole progenitor evolves through a substantial hydrogen-rich Wolf-Rayet (WNL) phase. Such evolution, which has been proposed by Langer (1987) for the most massive stars, keeps the helium core covered by hydrogen during most of the post-main sequence evolution, with the effect that it grows in mass instead of suffering extensive mass loss as hydrogen-free Wolf-Rayet star. We then discuss the important role of convective carbon burning, and its dependence on the C/O ratio resulting from core helium burning, for the iron core mass at the time of core collapse. We outline that the mass loss in hydrogen-free Wolf-Rayet stars leads to very high C/O ratios at core helium depletion. We argue that a high C/O ratio makes the formation of high-mass black holes less likely. We emphasize that in our scenario for Cyg X-1, an increased C/O ratio and ensuing convective carbon burning — as it occurs in models of very massive stars with a short WNL phase — is avoided. Our model for Cyg X-1 is similar to scenarios proposed for the origin of black hole transient sources, especially that of Nova Scorpii 1994. That is, it involves Preprint submitted to Elsevier Preprint 3 March 2008 a common envelope and spiral-in phase only after the black hole progenitor has evolved through a large fraction of core helium burning, so that its helium core can grow in mass. The difference is that in Cyg X-1 the companion star, an O-star, is much more massive than the companions in the transient sources. We expect an increasing number of high mass X-ray black hole binaries for lower metallicities.