Orbital capture of stars by a massive black hole via exchanges with compact remnants

We propose a dynamical mechanism for capturing stars around a massive black hole (MBH) that is based on the accumulation of a very dense cluster of compact stellar remnants near the MBH. This study is motivated by the presence of similar to10 young massive stars (M-* similar to3-15 M-., spectral types similar toB9 V - O8 V) less than 0.04 pc from the MBH in the Galactic center ( GC). Their existence in the extreme environment so close to an MBH is a challenge for theories of star formation and stellar dynamics. We show that young stars, which formed far from the MBH and were then scattered into eccentric orbits, repeatedly cross a cluster of stellar black holes (SBHs) where they may undergo rare direct three-body exchanges with an MBH-SBH "binary." The interaction between two objects of comparable mass ejects the SBH and captures the star on a tight orbit around the MBH. Such captures can naturally explain some trends observed in the orbits of the young stars. We derive the capture cross section, validate it by Monte Carlo simulations, and calculate the number of captured stars in the GC using the currently uncertain estimates of the numbers of SBHs in the inner 0.04 pc and of young stars in the inner few parsecs of the GC. We find that under favorable conditions, three-body exchange can account for similar to25% of the observed stars, mostly at the fainter end of the observed range. We discuss additional effects that possibly increase the capture efficiency. Future detections of the dark mass around the MBH and deeper surveys of the central parsecs will establish whether or not there are enough SBHs and young stars there for exchange captures to singly account for the central young stars. We estimate that there are also similar to35 lower mass stars (M-* similar to 1-3, M(.)similar toG2 V - A0 V) in the inner 0.04 pc similarly captured by exchanges with neutron stars (NSs). Ongoing replacement of compact remnants by mainsequence stars (SBHs by NS progenitors, NSs by white dwarf progenitors) may regulate the accumulation of compact remnants near the MBH.