Editors: Pau Amaro-Seoane & Bernard Schutz
The last GW Note is a Special Issues on eLISA/NGO

A star disrupted by a stellar black hole as the origin of the cloud falling towards the Galactic center


by Miralda-Escude, Jordi
submitted to ApJ

We propose that the cloud of gas moving on a highly eccentric orbit around the central black hole in our Galaxy, reported by Gillessen et al., is produced by a wind from photoevaporating debris orbiting around a star with a small circumstellar disk. The disk is tidally truncated to less than 1 AU at the peribothron passage, and a cloud like the observed one is recreated by the wind at every orbit. The star-disk system, which may have been producing the cloud for hundreds of orbits in the past, is proposed to have formed when the star flew by a stellar black hole and was tidally disrupted and deflected to its present orbit. Encounters of low-mass stars with stellar black holes are likely to occur at the location of this cloud, because of the high density of stellar black holes expected to have migrated to the Galactic center by mass segregation. The rate of these encounters at a small enough impact parameter to disrupt the star may reasonably be ~ 10^{-6} per year. The flyby should have spun up the star and pulled out a substantial fraction of its mass as tidal debris, part of which fell back onto the star and created a small disk. Since then, the disk may have expanded by absorbing angular momentum from the star up to the tidal truncation radius. Thereafter, the strong tidal perturbation of the outer disk edge at every peribothron may create gas streams moving out to larger radius that can photoevaporate and generate the wind that produces the cloud at every orbit. The model predicts that when the cloud is disrupted at the next peribothron passage in 2013, a smaller unresolved cloud will follow the star on the same orbit that will gradually grow. An increased infrared luminosity from the disk may also become detectable during the peribothron passage.

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