Correlation between mass segregation and structural concentration in relaxed stellar clusters

Abstract

The level of mass segregation in the core of globular clusters has been previously proposed as a potential indicator of the dynamical constituents of the system, such as presence of a significant population of stellar-mass black holes (BHs), or even a central intermediate-mass black hole (IMBH). However, its measurement is limited to clusters with high-quality Hubble Space Telescope data. Thanks to a set of state-of-the-art direct N-body simulations with up to 200k particles inclusive of stellar evolution, primordial binaries, and varying BH/neutron stars, we highlight for the first time the existence of a clear and tight linear relation between the degree of mass segregation and the cluster structural concentration index. The latter is defined as the ratio of the radii containing 5% and 50% of the integrated light (R 5 /R 50 ), making it robustly measurable without the need to individually resolve low-mass stars. Our simulations indicate that given R 5 /R 50 , the mass segregation ∆m (defined as the difference in main sequence median mass between center and half-light radius) is expressed as ∆m/M = −1.166R 5 /R h + 0.3246, with a root-mean-square error of 0.0148. In addition, we can explain its physical origin and the values of the fitted parameters through basic analytical modeling. Such correlation is remarkably robust against a variety of initial conditions (including presence of primordial binaries and IMBHs) and cluster ages, with a slight dependence in best-fit parameters on the prescriptions used to measure the quantities involved. Therefore, this study highlights the potential to develop a new observational tool to gain insight on the dynamical status of globular clusters and on its dark remnants.

Dynamically formed black hole binaries: In-cluster versus ejected mergers

“…In this work, we analyse the mergers of BHBs formed within simulated GCs, comparing the binary parameters of systems which merge inside their host cluster, to systems which merge after being ejected. The analysis is based on a novel set of direct NBODY6 simulations of realistic cluster models spanning a wide range of initial conditions, partially presented previously by de Vita et al (2019) in the context of structural GC properties. The simulations include SSE and BSE, galactic tides, gravitational radiation, and other relativistic effects, making them an ideal tool to explore BHB mergers and compare with recent LIGO results.…”

“…In this paper, we resort to a new large set of direct N-body simulations designed to investigate properties of compact object mergers (de Vita, Trenti, & MacLeod 2019;MacLeod, Trenti, & Ramirez-Ruiz 2016) to focus on the formation of BHBs within mid-sized star clusters containing 5 × 10 4 N 2 × 10 5 stars, a range that includes the 8.1 × 10 4 M median mass for Milky Way GCs (see Heggie & Hut 2003 Table 1.1) under a typical stellar initial mass function (IMF) even after accounting for mass loss during dynamical evolution. We compare the properties of binaries that merge within a cluster to systems that merge after they are ejected from the cluster, with the goal of understanding how the channels are represented in current and future LIGO observations.…”

A Dynamical Survey of Stellar-Mass Black Holes in 50 Milky Way Globular Clusters

“…Under these constraints, we chose a high-mass population containing all stars with L > L TO and a low-mass population consisting of MS stars with L TO /125 ≤ L ≤ L TO /25. While these population choices (Pop1 and Pop4 in Figure 1) maximized the magnitude of ∆ while ensuring relatively large observable population sizes, reducing statistical scatter compared to other choices in previous studies (e.g., blue stragglers; Peuten et al 2016;Alessandrini et al 2016), they were not free from drawbacks (de Vita et al 2019). Specifically, Pop1 contains far fewer stars than any of the three MS populations, introducing higher statistical scatter than strictly necessary.…”

Structure and Internal Kinematics of Nine Inner Milky Way Globular Clusters

“…Libralato et al 2018;Watkins et al 2020), which our data quality unfortunately does not currently permit. With this value in hand, the core-collapsed nature of any cluster can be independently verified using only internal kinematics (Libralato et al 2018;Bianchini et al 2018b), while allowing a test of the predicted empirical relationship between concentration and mass segregation (de Vita et al 2019), especially important in light of current discrepancies between theory and observations for two well-studied core-collapsed GGCs (Cadelano et al 2020b). More generally, simulations predict that dynamically evolved (t 10 t rc ) clusters should have low mass-to-light ratios of M/L 3 inside r hl (and post-corecollapse dynamical evolution will act to lower the global mass-to-light ratio), and further predict that the exact values of M/L are metallicity dependent (Bianchini et al 2017b, but see Dalgleish et al 2020).…”