Studies of the observed characteristics of black-hole (BH) X-ray binaries can
be provide us with valuable information about the process of BH formation. In
this paper I address some of the aspects of our current understanding of BH
formation in binaries and point out some of the existing problems of current
theoretical models. In particular, the measured orbital periods and donor-star
properties indicate that a common-envelope phase appears to be a necessary
ingredient of the evolutionary history of observed BH X-ray transients, and
that it must be associated only with a modest orbital contraction. The timing
of this common-envelope phase is crucial in determining the final BH masses and
current evolutionary models of mass-losing massive stars place strong
constraints on the possible masses for immediate BH progenitors and wind mass
loss from helium stars. Last, it is interesting that, even in the absence of
any source of mass loss, the highest helium-star masses predicted by current
evolutionary models are still not high enough to account for the measured BH
mass in V404 Cyg (>10 solar masses). An alternative for the formation of
relatively massive BH may be provided by the evolutionary sequence proposed by
Eggleton & Verbunt (1986), which invokes hierarchical triples as progenitors of
BH X-ray binaries with low-mass companions.Comment: 10 pages, 3 figures, to appear in Evolution of Binary and Multiple
Star Systems, ASP Conf.Series, 2001, P. Podsiadlowski et al. (eds.)
(proceedings from a Meeting in Celebration of Peter Eggleton's 60th Birthday