Populating the Upper Black Hole Mass Gap through Stellar Collisions in Young Star Clusters

Abstract

Theoretical modeling of massive stars predicts a gap in the black hole (BH) mass function above ∼40-50 M e for BHs formed through single star evolution, arising from (pulsational) pair-instability supernovae (PISNe). However, in dense star clusters, dynamical channels may exist that allow construction of BHs with masses in excess of those allowed from single star evolution. The detection of BHs in this so-called "upper-mass gap" would provide strong evidence for the dynamical processing of BHs prior to their eventual merger. Here, we explore in detail the formation of BHs with masses within or above the pair-instability gap through collisions of young massive stars in dense star clusters. We run a suite of 68 independent cluster simulations, exploring a variety of physical assumptions pertaining to growth through stellar collisions, including primordial cluster mass segregation and the efficiency of envelope stripping during collisions. We find that as many as ∼20% of all BH progenitors undergo one or more collisions prior to stellar collapse and up to ∼1% of all BHs reside within or above the pair-instability gap through the effects of these collisions. We show that these BHs readily go on to merge with other BHs in the cluster, creating a population of massive BH mergers at a rate that may compete with the "multiple-generation" merger channel described in other analyses. This has clear relevance for the formation of very massive BH binaries as recently detected by the Laser Interferometer Gravitational-Wave Observatory/Virgo in GW190521. Finally, we describe how stellar collisions in clusters may provide a unique pathway to PISNe and briefly discuss the expected rate of these events and other electromagnetic transients.

Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star Clusters

“…In the case of extremely efficient energy release in the form of a jet, a bright X-ray or γray flare may result with the overall phenomenology possibly resembling ultra-long gamma-ray bursts (Perets et al 2016). These TDEs are expected to occur in GCs at rates of roughly 3-10 Gpc −3 yr −1 (Perets et al 2016;Lopez et al 2020;Kremer et al 2019c) and at similar rates in both nuclear star clusters (Fragione et al 2020) and in stellar triples under the influence of Lidov-Kozai oscillations (Fragione et al 2019).…”

“…BH-star encounters are expected to play a crucial role in the formation of both accreting and detached BH binaries (e.g., Ivanova et al 2010Ivanova et al , 2017Giesler et al 2018;Kremer et al 2018a) with properties similar to the BH candidates detected to date in Milky Way GCs (e.g., Kremer et al 2019a). Additionally, such dynamical encounters may occasionally cause a star to cross a BH within its tidal disruption radius, leading to a tidal disruption of the star (Perets et al 2016;Lopez et al 2020;Kremer et al 2019bKremer et al , 2019cSamsing et al 2019;Fragione et al 2020). These stellar-mass BH tidal disruption events (TDEs) may occur during close encounters of pairs of single stars (i.e., single-single interactions) and also during small-N (typically three-or four-body) resonant encounters that occur through binary-mediated dynamical interactions (e.g., Fregeau & Rasio 2007).…”

“…In reality, especially if the TDE occurs during a multi-body resonant encounter, TDEs may affect the hydrodynamic evolution of their dynamical encounters. These more complex effects are well beyond the computational scope of an N-body code like CMC, but see e.g., Lopez et al (2020) for a discussion. We apply this TDE prescription only for BH-star interactions.…”

“…For close encounters of star-star pairs, we allow the stars to interact only in the direct collision limit: we assume a sticky sphere collision (i.e., zero mass loss) occurs if r p < R 1 + R 2 , where R 1 and R 2 are the stellar radii. See Kremer et al (2020a) for further details of our treatment of star-star collisions. We record TDEs/collisions that occur during both single-single encounters and binary-mediated dynamical encounters that are integrated directly using Fewbody.…”

Black Hole Mergers from Star Clusters with Top-heavy Initial Mass Functions

“…Second, for initial N held constant, clusters with top-heavy IMFs also have greater total mass and higher collision rates in their densely populated cores. This collision rate enhancement is obvious in Table 1, column 7, listing the number of mass gap BBH merger components formed from the stellar product of collisions (e.g., Di Carlo et al 2020;Kremer et al 2020c). While the column only lists collisionally formed merger components in the mass gap, the overall collision rate scales similarly with decreasing α 3 .…”