How Initial Size Governs Core Collapse in Globular Clusters

Abstract

Globular clusters (GCs) in the Milky Way exhibit a well-observed bimodal distribution in core radii separating the so-called "core-collapsed" and "non-core-collapsed" clusters. Here, we use our Hénon-type Monte Carlo code, CMC, to explore initial cluster parameters that map into this bimodality. Remarkably, we find that by varying the initial size of clusters (specified in our initial conditions in terms of the initial virial radius, r v ) within a relatively narrow range consistent with the measured radii of young star clusters in the local universe (r v ≈ 0.5 − 5 pc), our models reproduce the variety of present-day cluster properties. Furthermore, we show that stellar-mass black holes (BHs) play an intimate role in this mapping from initial conditions to the present-day structural features of GCs. We identify "best-fit" models for three GCs with known observed BH candidates, NGC 3201, M22, and M10, and show that these clusters harbor populations of ∼ 50 − 100 stellar-mass BHs at present. As an alternative case, we also compare our models to the core-collapsed cluster NGC 6752 and show that this cluster likely contains few BHs at present. Additionally, we explore the formation of BH binaries in GCs and demonstrate that these systems form naturally in our models in both detached and mass-transferring configurations with a variety of companion stellar types, including low-mass main sequence stars, white dwarfs, and sub-subgiants.

Modeling Dense Star Clusters in the Milky Way and Beyond with the CMC Cluster Catalog

“…Thus, by varying r v (here, from 0.5-4 pc) the full distribution of cluster core radii is naturally captured. This same result was demonstrated in Kremer et al (2019a) for a much smaller set of models of fixed particle number and metallicity. Here, we demonstrate this result for a much broader set of cluster properties.…”

“…Much of the recent theoretical work on GC dynamics has focused on the crucial role played by stellar black hole (BH) remnants in these systems. Several studies have shown that large numbers of stellar-mass BHs can be retained in typical GCs all the way to the present, and their dynamical interactions in the cluster core over its ∼ 12 Gyr of evolution provide a natural physical explanation for many of the diverse cluster features alluded arXiv:1911.00018v1 [astro-ph.HE] 31 Oct 2019 to in the previous paragraph (e.g., Morscher et al 2015;Mackey et al 2008;Breen & Heggie 2013;Askar et al 2018;Kremer et al 2019a). The first BH candidate in a cluster was identified in the extragalactic GC NGC 4472 by Maccarone et al (2007) through X-ray observations.…”

“…In Kremer et al (2019a), we demonstrated that the initial cluster size (set as the initial virial radius, r v ) is the key parameter which determines the ultimate fate of a cluster and its BH population. A cluster's half-mass relaxation time is related to its virial radius through the expression…”

“…Thus, clusters with smaller initial r v have shorter relaxation times and are thus more dynamically evolved at their present age (t ∼ 12 Gyr) compared to clusters born with larger initial r v . In Kremer et al (2019a), we employed a small set of cluster simulations with a number of initial parameters, such as total particle number and metallicity, fixed to reflect the median values of the clusters observed in the MW. In that analysis, we developed best-fit models for a set of four MW clusters (NGC 3201, M10, M22, and NGC 6752) by matching various observed features (including surface brightness and velocity-dispersion profiles).…”

“…In that analysis, we developed best-fit models for a set of four MW clusters (NGC 3201, M10, M22, and NGC 6752) by matching various observed features (including surface brightness and velocity-dispersion profiles). Here, we expand upon the results of Kremer et al (2019a) and explore the effect of initial virial radii, and subsequent BH dynamics, on clusters of various masses, metallicity, and locations within the Galactic tidal field. We develop a grid of 148 independent cluster simulations, run using CMC (for Cluster Monte Carlo) which covers roughly the complete range of GCs observed at present in the MW.…”