Orbital Capture of Stars by a Massive Black Hole via Exchanges with Compact Remnants

Abstract

We propose a dynamical mechanism for capturing stars around a massive black hole (MBH), which is based on the accumulation there of a very dense cluster of compact stellar remnants. This study is motivated by the presence of ~10 young massive stars (M*~3-15 Mo, ~B9V--O8V) less than 0.04 pc from the MBH in the Galactic center (GC). Their existence in the extreme environment so close to a 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 3-body exchanges with a MBH-SBH "binary". The interaction between two objects of similar mass ejects the SBH and captures the star on a tight orbit around the MBH. Such captures can naturally explain 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 inside 0.04 pc and of young stars in the inner few pc of the GC. We find that under favorable conditions 3-body exchange can account for ~25% of the observed stars, mostly at the fainter end of the observed range. We discuss additional effects that possibly increase the capture efficiency. Future observations will establish whether there are enough SBHs and young stars for exchange captures to singly account for the central young stars. We estimate that there are also ~35 lower mass stars (M*~1-3 Mo, ~G2V--A0V) inside 0.04 pc similarly captured by exchanges with neutron stars. Ongoing replacement of compact remnants by main sequence stars may regulate the accumulation of compact remnants near the MBH.Comment: ApJL in press. 4 pp, 1 fig (final version; typo fixed in eq. 1; refs updated