http://arxiv.org/abs/1312.5924
There are 19 confirmed black hole binaries in the Galaxy. 16 of them are X-ray transients hosting a ~5-15 Msun black hole and a Roche lobe overflowing low mass companion. Companion masses are found mostly in 0.1-1 Msun mass range with peak at 0.6 Msun. The formation of these systems is believed to involve a common envelope phase, initiated by a black hole progenitor, expected to be a rather massive star >20 Msun. We show that none of the available common envelope models allow for the formation of the observed population of Galactic black hole transients. Black holes are most likely found with companions of mass 1 Msun for the standard CE energy prescription. The companion mass distribution also peaks at 1Msun and is still in tension with observations even if the envelope binding energy is lowered by factor 5 as suggested by Ivanova and Chaichenets (2011). The angular momentum CE prescription produces rather flat companion mass distribution over a broad range 0.5-1.3 Msun with significantly less systems than observed in the Galaxy. Additionally, we have modified the most important factors shaping the companion mass distribution: magnetic braking, mass transfer rate and star formation rate. In each case almost no companions are found around the observed mass peak. We argue that common envelope physics is not crucial in the understanding of Galactic BH transients. Our failure most likely indicates that either the current evolutionary models for low mass stars and magnetic braking are not realistic or that the intrinsic population of black hole transients is quite different from the observed one.
Mon, 23 Dec 13
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