Catch me if you can: Is there a "runaway-mass" black hole in the orion nebula cluster?

Abstract

We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by means of direct N-body integrations. A large fraction of residual gas was probably expelled when the ONC formed, so we assume that the ONC was much more compact when it formed compared with its current size, in agreement with the embedded cluster radius-mass relation from Marks & Kroupa. Hence, we assume that few-body relaxation played an important role during the initial phase of evolution of the ONC. In particular, three-body interactions among OB stars likely led to their ejection from the cluster and, at the same time, to the formation of a massive object via "runaway" physical stellar collisions. The resulting depletion of the high-mass end of the stellar mass function in the cluster is one of the important points where our models fit the observational data. We speculate that the runaway-mass star may have collapsed directly into a massive black hole (M• ≳ 100 M ⊙). Such a dark object could explain the large velocity dispersion of the four Trapezium stars observed in the ONC core. We further show that the putative massive black hole is likely to be a member of a binary system with ≈70% probability. In such a case, it could be detected either due to short periods of enhanced accretion of stellar winds from the secondary star during pericentre passages, or through a measurement of the motion of the secondary whose velocity would exceed 10 km s-1 along the whole orbit. © 2012. The American Astronomical Society. All rights reserved.