HOME

About Me

CV

Publications

Abstract

Studying black hole formation channels enabled by dense environments, such as star clusters, is crucial for understanding the formation of bodies that would otherwise not be possible through isolated stellar evolution. Theoretical evolution models predict a "gap" in the black hole mass spectrum between 40-120 M_☉caused by pair-instability supernovae. Here we investigate whether black holes with masses that fall within this "upper mass gap" (similar to the recent LIGO/Virgo event GW190521) can be formed through different formation channels enabled by dense clusters. In particular, we explore the roles of the high-mass binary fraction and cluster density and their repercussions on black hole growth. We perform a set of simulations using the Cluster Monte Carlo (CMC) code and study the formation and collision histories for the most massive stars that form. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. The results show a strong argument for the formation of massive black holes via dynamical processing as well as binary coalescences in dense clusters.

For a long discussion check our recent publication!

Description

Clusters are dense environments in which stellar evolution is not necessarily an isolated process but is rather affected by dynamical interactions. These dynamical interactions between bodies create different formation channels that allow for the formation of black holes with masses exceeding those theorized by isolated stellar evolution models. Several studies have shown that pair-instability supernovae, a thermonuclear explosion that leaves no stellar remnant, can prevent the formation of black holes with masses in the range of 40-120 M_☉(Spera & Mapelli (2017)). However, this well-known "upper mass gap" in the black hole mass spectrum could be potentially populated by remnants of massive stars that have experienced dynamical processing in dense clusters (Kremer et al. (2020a) ). In particular, mergers between a main sequence star and an evolved star would lead to a helium core that could survive the supernovae explosions and result in pair instability gap black holes. Figure 1 illustrates this formation path, where an initial binary-binary interaction leads to the merger of the component masses of one of the binaries. The merger product then experiences some dynamical collisions before collapsing into a black hole of 89M_☉. Such black holes are of great value for the gravitational wave field since mergers between them and a stellar black hole would be detected by the LIGO/Virgo Interferometers.

An important parameter of star clusters is the primordial binary fraction of massive stars. Studies have shown that nearly 100% of O- aand B-type stars in the Galactic field are in binaries (Sana et al. (2012) ). However, for star clusters, the primordial binary fraction of massive stars is less well constrained. Binaries have been shown to play a significant role in producing high rates of both stellar collisions (Fregeau & Rasio et al. (2007) ) and BH mergers (Chartterjee et al. (2017) ). Thus, it is crucial to constraint the primordial binary fraction to fully understand and model cluster evolution.

Models of cluster evolution

We perform the set of simulations using Cluster Monte Carlo (CMC) which models the evolution of stellar clusters. See Kremer et al. (2020b) for a summary of CMC. This code incorporates COSMIC, which simulates compact object and stellar binaries (refer to Breivik et al. (2019) for a detailed description) as well as Fewbody, a numerical toolkit that performs scattering experiments (refer to Fregeau et al. (2004) for a detailed description). See Kremer et al. (2020a) for a brief discussion on the treatment of collision products, collision runaways and compact object formation for this set of simulations.

We study the parameter space spanned by variations in both the virial radius and the high-mass binary fraction, which in this study we define as the fraction of masses above 15 M_☉ that are in a binary at the time of cluster formation. The simulations performed include values for virial radius of 1, 1.2 and 1.5 pc and high-mass binary fraction of 0 and 1. For all models, the low-mass binary fraction is kept at 0.05. Furthermore, the simulations consist of 8 × 10⁵ objects and a total cluster mass of 4.7 × 10⁵ M_☉. The simulations are run for a total of 30 Myr, as we are only interested in black hole growth, which happens in this time span. The metallicity is set to 0.002 (0.1 Z_☉) and we adopt a Kroupa IMF in the range of 0.08 - 150 M_☉. We run multiple realizations of each set of initial parameters with different random seeds to increase the robustness of our results.

Results

We discuss briefly some of the most important results of this study, but you should refer to the publication for a more thorough analysis. The top panel in Figure 2 shows the normalized black hole mass spectrum comparison between the two models with high mass binary fraction 0 and 1 and a virial radius of 1pc. The blue background marks the assumed boundaries for the pair-instability mass gap. As can be seen, the models with binary fraction of 1 produce significantly more black holes in the upper mass gap as well as some intermediate-mass black holes. From this, it is clear that the high-mass binary fraction makes a strong imprint in the black hole mass spectrum. Furthermore, the bottom panel plots the normalized black hole spectrum for the different values of virial radius (we use the initial virial radius of the assumed King model to determine the initial cluster density) and a fixed high-mass binary fraction of 1. It is clear that more compact clusters (smaller virial radius) leads to an increase in the rate of dynamical collisions between massive stars which in turn produces more massive BHs. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants easily detectable as gravitational wave sources by LIGO/Virgo or future gravitational wave detectors.

Future Work

A detailed study of the long term evolution of these simulations is needed, including long-term dynamics of massive black holes.

Acknowledgments

This material is based upon work supported by the National Science Foundation under grant No. AST-1757792. This work used computing resources at CIERA.