The survival of star clusters with black hole subsystems

Abstract

Recent observations have detected top-heavy IMFs in dense star forming regions like the Arches cluster. Whether such IMFs also exist in old dense stellar systems like globular clusters is difficult to constrain, because massive stars already became black holes (BHs) and neutron stars (NSs). However, studies of stellar dynamics find that BHs/NSs influence the long-term evolution of star clusters. Following Breen & Heggie (2013) and by carrying out two-component N -body simulations, we demonstrate how this dynamical impact connects with the shape of IMFs. By investigating the energy balance between the BH subsystem and the global, we find that to properly describe the evolution of clusters, a corrected two-body relaxation time, T rh,p = T rh /ψ, is necessary. Because ψ depends on the total mass fraction of BHs, M 2 /M , and the mass ratio, m 2 /m 1 , the cluster dissolution time is sensitive to the property of BHs or IMFs. Especially, the escape rate of BHs via ejections from few-body encounters are linked to mass segregation. In strong tidal fields, top-heavy IMFs easily lead to the fast dissolution of star clusters and the formation of BH-dominant dark clusters, which suggests that the observed massive GCs with dense cores are unlikely to have extreme top-heavy IMFs. With the future observations of gravitational waves providing unique information of BHs/NSs, it is possible to combine the multi-message observations to have better constrains on the IMFs of old star clusters.

Merger rate of black hole binaries from globular clusters: Theoretical error bars and comparison to gravitational wave data from GWTC-2

“…Because the relaxation time decreases if the mass reduces, including this type of mass evolution will lead to a higher merger rate than in our models with anṀ that is constant in time. The models of [51] also do not contain BHs and it has been shown that retaining a BH population significantly increases the escape rate of stars [107,108]. The BH population can increase |Ṁ | by an order magnitude (Gieles et al, in prep), especially towards the end of the evolution.…”

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

“…On the theoretical side, stateof-the-art N-body modeling has shown that stellar-mass BHs form and are retained in the stellar clusters of all masses (e.g., Morscher et al 2015;Wang et al 2015;Banerjee 2017;Arca Sedda et al 2018;Askar et al 2018;Weatherford et al 2020;Kremer et al 2020b). Furthermore, cluster simulations have revealed that these BHs play a crucial role in the long-term dynamics, core evolution, and survival of stellar clusters (e.g., Mackey et al 2007;Breen & Heggie 2013;Chatterjee et al 2017;Kremer et al 2018bKremer et al , 2019aKremer et al , 2020bYe et al 2019;Giersz et al 2019;Wang 2020).…”

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

“…Observations provide evidence that the stellar IMF may not be universal. A noncanonical IMF can greatly affect a star cluster's dynamical evolution, especially since its high end determines how many BHs form within, regulating the cluster's energy budget and dynamical clock (Breen & Heggie 2013;Chatterjee et al 2017;Kremer et al 2020a;Wang 2020). In this Letter, we have extended the CMC Cluster Catalog (Kremer et al 2020b) to examine how a varying IMF affects the evolution of star clusters and their BH populations.…”

“…During these events, the BHs mix with the rest of the cluster and provide energy to passing stars in scattering interactions (Breen & Heggie 2013). Cumulatively, these interactions inflate the cluster halo and force faster evaporation through the tidal boundary (Chatterjee et al 2017;Giersz et al 2019;Wang 2020).…”

“…The impact on cluster evolution of varying BH abundance has recently been further analyzed via a combination of analytical calculations and N-body simulations (Breen & Heggie 2013;Wang 2020). However, these studies only consider idealized star clusters with two components (stars and BHs) and no stellar evolution, which plays a crucial role in cluster disruption (e.g., Chernoff & Weinberg 1990).…”