Binary Black Holes in Dense Star Clusters: Exploring the Theoretical Uncertainties

Abstract

Recent N-body simulations predict that large numbers of stellar black holes (BHs) could at present remain bound to globular clusters (GCs), and merging BH-BH binaries are produced dynamically in significant numbers. We systematically vary "standard" assumptions made by numerical simulations related to, e.g., BH formation, stellar winds, binary properties of high-mass stars, and IMF within existing uncertainties, and study the effects on the evolution of the structural properties of GCs, and the BHs in GCs. We find that variations in initial assumptions can set otherwise identical initial clusters on completely different evolutionary paths, significantly affecting their present observable properties, or even affecting the cluster's very survival to the present. However, these changes usually do not affect the numbers or properties of local BH-BH mergers. The only exception is that variations in the assumed winds and IMF can change the masses and numbers of local BH-BH mergers, respectively. All other variations (e.g., in initial binary properties and binary fraction) leave the masses and numbers of locally merging BH-BH binaries largely unchanged. This is in contrast to binary population synthesis models for the field, where results are very sensitive to many uncertain parameters in the initial binary properties and binary stellar-evolution physics. Weak winds are required for producing GW150914-like mergers from GCs at low redshifts. LVT151012 can be produced in GCs modeled both with strong and weak winds. GW151226 is lower-mass than typical mergers from GCs modeled with weak winds, but is similar to mergers from GCs modeled with strong winds.

Black hole binaries dynamically formed in globular clusters

“…One of the interesting implications of their work is that the majority of BBHs would have mass ranges between 32 − 66 M⊙, and the contribution of cluster-origin BBHs would be roughly 14 per cent among the all merging BBHs in the local Universe. Adapting similar Monte-Carlo models, Chatterjee et al (2017) examined how different initial assumptions affect the cluster's evolution and properties of BH populations. They concluded that initial mass function, stellar wind, natal kicks are most important factors to affect the number of BBHs or properties of BH populations formed in clusters.…”

Dynamical Formation of Low-mass Merging Black Hole Binaries like GW151226

“…Variations in initial N and r v capture both of these effects. While the merger rates do depend on other variations, e.g., in the initial binary fraction and binary orbital properties of high-mass stars, the stellar IMF, and the distribution of natal kicks, the mass distribution of BBHs merging in the local universe is insensitive to these variations (Chatterjee et al 2017). Figure 2.…”

“…The process involved here is fundamentally different from BBH formation in isolation. A negligible fraction of BBHs formed in dense massive star clusters are primordial, and none with t delay 1 Gyr are composed of BHs formed from stars that were born in that binary (e.g., Chatterjee et al 2017). As massive stellar binaries evolve in dense star clusters, even if they were initially hard, mass loss from stellar winds and compact object formation can make these binaries soft.…”

“…Because of this, the BBH properties and their merger times, unlike those formed in isolation, do not depend on the assumptions of initial binarity or binary orbital properties. While the rate of mergers is affected by the assumptions for the IMF and natal kicks, the BBH masses and merger delay times (t delay ) are insensitive to those as well (Chatterjee et al 2017).…”

“…While several studies have modeled BBH formation in isolation for a wide range in metallicities taking into account the cosmological evolutions of the star formation rate (SFR) and metallicity (e.g., Belczynski et al 2016a;Dvorkin et al 2016), due to primarily the computational cost, numerical studies of dynamically formed BBHs have so far restricted themselves to either a narrow range in metallicities and ages typical of the Galactic globular clusters (GGCs; e.g., Rodriguez et al 2015;Chatterjee et al 2017) or to low-mass (initial N∼5×10…”