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Abstract
Abstract We describe the public release of the Cluster Monte Carlo (CMC) code, a parallel, star-by-star N-body code for modeling dense star clusters. CMC treats collisional stellar dynamics using Hénon’s method, where the cumulative effect of many two-body encounters is statistically reproduced as a single effective encounter between nearest-neighbor particles on a relaxation timescale. The star-by-star approach allows for the inclusion of additional physics, including strong gravitational three- and four-body encounters, two-body tidal and gravitational-wave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release of CMC is pinned directly to the COSMIC population synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest (N = 108) star-by-star N-body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pair-instability mass gap).
Highlights
The modeling of dense star clusters (DSCs), such as globular clusters (GCs), super star clusters (SSCs), or the nuclear star clusters (NSCs) in the centers of many galaxies, remains one of the most challenging problems in computational astrophysics
We present two examples of star cluster modeling: first, we perform the largest (N = 108) star-by-star N -body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution
While COSMIC was originally based on the version of Binary Stellar Evolution (BSE) incorporated into Cluster Monte Carlo (CMC), including its updates to compact object physics (Chatterjee et al 2010; Rodriguez et al 2016), the two codes have diverged over the past several years, and the use of COSMIC as a communitydriven population synthesis code has kept it up-to-date with the latest developments in binary stellar evolution
Summary
The first approach employed to study collisional stellar dynamics has often been a direct summation approach. Even with dozens of parallel GPUs, these models require months or even years of wall-clock time to integrate a cluster for ∼ 10Gyr, (e.g., Heggie 2014; Wang et al 2016; Rantala et al 2021), and are limited to systems with low binary fractions, large initial radii, and relatively long dynamical times These constraints preclude any reasonable exploration of the parameter space of massive star clusters, especially those with the compact initial radii needed to produce core-collapsed GCs There are key differences in the choices of the dynamical timestep and other aspects of stellar evolution and collisions, CMC and MOCCA contain much of the same physics, and are currently the only codes capable of modeling realistic populations of DSCs with more than 106 stars and binaries over many Gyr. In this paper, we describe the first public release of the Cluster Monte Carlo code, CMC, an N -body approach to modeling collisional stellar dynamics with Henon’s method.
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