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 per cent 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 per cent 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 a MSBH.

The Close Stellar Companions to Intermediate-Mass Black Holes

“…This outcome is similar to the results from Mapelli & Zampieri (2014) for systems undergoing Roche lobe overflow. Companion objects include BHs (1-2 Gyr), as well as main sequence (MS) and giant-branch stars, and white dwarfs (WD) of various masses.…”

“…Konstantinidis et al (2013) argue that the merger generates sufficient gravitational wave recoil to eject the IMBH from their low-mass simulation cluster. Mapelli et al (2013c), Mapelli & Zampieri (2014), and Ziosi et al (2014) study the dynamics of massive stellar-mass BHs (up to 10-100 M e ) in a large sample of N-body calculations of young star clusters with a range of metallicities and N=5500 stars. Leigh et al (2014) also run N-body simulations containing IMBHs with between N=32k and N=131k stars, but they do not include gravitational radiation from compact binaries.…”

“…The trend toward energy equipartition in two-body interactions causes stars more massive than the mean mass to sink toward orbits deep in the cluster core, where they are very likely to come into direct interaction (and potentially partnership) with the IMBH (e.g., Gürkan et al 2004;Goswami et al 2012;Leigh et al 2014). Further, more massive secondary objects are likely to replace an existing companion if they do undergo a three-body interaction (Fullerton & Hills 1982;Sigurdsson & Phinney 1993;Mapelli & Zampieri 2014;Samsing et al 2014). In the lower right panel of Figure 4, we show the unnormalized distribution of companion masses in order to demonstrate that there is strong evolution in the companion mass distribution with stellar population age.…”

“…Through dynamical interaction, these BHs could acquire and accrete mass from companion objects (Mapelli et al 2013c;Mapelli & Zampieri 2014;Ziosi et al 2014), which implies a self-consistent channel by which the BH could grow into the IMBH mass range as the cluster evolves.…”

Dissipative Evolution of Unequal-mass Binary–single Interactions and Its Relevance to Gravitational-wave Detections

“…We speculate that this could represent the dynamical environment of an early collapse associated with the formation of a dense stellar system, such as a GC (e.g. Mapelli et al 2013;Mapelli & Zampieri 2014;Ziosi et al 2014;Spera et al 2015;Kimpson et al 2016). For this scenario we study the cumulative number of BBH mergers forming through the GW inspiral channel, N Iij , relative to that of the post-interaction binary merger channel, N Mij , as a function of time τ = t − t 0 .…”