Roche-lobe Overflow Systems Powered by Black Holes in Young Star Clusters: The Importance of Dynamical Exchanges

Abstract

We have run 600 N-body simulations of intermediate-mass (~3500 M _⊙) young star clusters (SCs; with three different metallicities (Z = 0.01, 0.1, and 1 Z _⊙). The simulations include the dependence of stellar properties and stellar winds on metallicity. Massive stellar black holes (MSBHs) with mass >25 M _⊙ are allowed to form through direct collapse of very massive metal-poor stars (Z < 0.3 Z _⊙). We focus on the demographics of black hole (BH) binaries that undergo mass transfer via Roche lobe overflow (RLO). We find that 44% of all binaries that undergo an RLO phase (RLO binaries) formed through dynamical exchange. RLO binaries that formed via exchange (RLO-EBs) are powered by more massive BHs than RLO primordial binaries (RLO-PBs). Furthermore, the RLO-EBs tend to start the RLO phase later than the RLO-PBs. In metal-poor SCs (0.01-0.1 Z _⊙), >20% of all RLO binaries are powered by MSBHs. The vast majority of RLO binaries powered by MSBHs are RLO-EBs. We have produced optical color-magnitude diagrams of the simulated RLO binaries, accounting for the emission of both the donor star and the irradiated accretion disk. We find that RLO-PBs are generally associated with bluer counterparts than RLO-EBs. We compare the simulated counterparts with the observed counterparts of nine ultraluminous X-ray sources. We discuss the possibility that IC 342 X-1, Ho IX X-1, NGC 1313 X-2, and NGC 5204 X-1 are powered by an MSBH.