On the Formation of Low-Mass Black Holes in Massive Binary Stars

Recently, Brown & Bethe suggested that most stars with main-sequence mass in the range of ∼18-30 Msun explode, returning matter to the Galaxy, and then go into low-mass (≥ 1.5 Msun) black holes. Even more massive main-sequence stars would chiefly go into high-mass (∼10 Msun) black holes. The Brown-Bethe estimates gave ∼15 x 108 low-mass black holes in the Galaxy. We here address why none of these have been seen, with the possible exception of the compact objects in SN 1987A and 4U 1700-37. Our main point is that the primary star in a binary loses its hydrogen envelope by transfer of matter to the secondary and loss into space, and the resulting "naked" helium star evolves differently than a helium core, which is at least initially covered by the hydrogen envelope in a massive main-sequence star. We show that primary stars in binaries can end up as neutron stars even if their initial mass substantially exceeds the mass limit for neutron star formation from single stars (∼118 Msun). An example is 4U 1223-62, in which we suggest that the initial primary mass exceeded 35 Msun, yet X-ray pulsations show a neutron star to be present. We also discuss some individual systems and argue that 4U 1700-37, the only example of a well-studied high-mass X-ray binary that does not pulse, could well contain a low-mass black hole. The statistical composition of the X-ray binary population is consistent with our scenario, but due to the paucity of systems it is consistent with more traditional models as well.