A binary black hole (BBH) with components of 30-40 solar masses as the source of gravitational waves GW150914 can be formed from a relatively isolated binary of massive stars if both BHs are formed by implosion, namely, by complete or almost complete collapse of massive stars with no energetic SNe accompanied by a sudden mass loss that would significantly reduce the mass of the compact objects, and in most cases unbind the binary system. BBHs can also be formed by dynamical interactions in globular clusters, if the BHs are formed with no energetic SNe that would kick the BHs out from the cluster before BBH formation. Besides, if BHs of ~10 solar masses as in the source GW151226 are formed by implosion, the formation of BBHs would be prolific, and their fusion would make an important contribution to a stochastic gravitational wave background. Theoretical models set mass ranges for BH formation by implosion, but until recently observational evidences had been elusive. Here are reviewed the observational insights on the formation of BHs by implosion obtained by: (1) the kinematics in 3D of Galactic BH X-ray binaries that allow to set limits to putative natal SN kicks to BHs, (2) the absence in archived images of stellar progenitors of core-collapse SNe above 18 solar masses, (3) the identification of luminous stars that disappear without optically bright SNe, (4) the absence in the nebular spectra of type IIP SNe of nucleosynthetic products from progenitors above 20 solar masses, (5) the detection of gravitational waves from BBHs. From the results in these areas of observational astrophysics, and the recently confirmed dependence of BH formation on metallicity and redshift, it is concluded that a large fraction of massive stellar binaries in the universe end as BBHs, and that BHs of ~10 solar masses and even lower masses may be formed by implosion.
Wed, 28 Sep 16
Comments: 21 pages, 10 figures, Invited Review submitted to New Astronomy Reviews