New parameters for star cluster dynamics: the effect of primordial binaries and dark remnants

TL;DR

This study extends the analysis of star cluster dynamics by introducing parameters to quantify evolutionary stages and examines how primordial binaries and black holes influence these parameters and the cluster's evolution.

Contribution

It introduces and validates three parameters to track cluster evolution, considering the effects of primordial binaries and black holes on dynamical history.

Findings

Higher binary fractions lead to milder contraction and shallower core collapse.

Retention and ejection timescales of black holes significantly affect cluster structure and parameter evolution.

Parameters can indirectly indicate the presence of black holes in clusters.

Abstract

By studying the normalized cumulative radial distribution (nCRD) of the stars in the central region of a Monte Carlo-simulated globular cluster, we recently defined three parameters able to pinpoint the stage of internal dynamical evolution reached by the system: $A_5$ (i.e., the area subtended by the nCRD within 5$\%$ the half-mass radius, $r_h$), $P_5$ (the value of the nCRD at 5$\%$ $r_h$), and $S_{2.5}$ (the slope of the nCRD at 2.5$\%$ $r_h$). Here we extend the analysis and explore the effects that different fractions (0$\%$, 10$\%$, and 20$\%$) of primordial binaries and stellar-mass black holes (BHs) induce on the dynamical history of the system. As expected, the gradual contraction of the cluster becomes milder and core collapse shallower for increasing binary fraction. Nevertheless, the cluster dynamical evolution is still properly traced by the three parameters. For models with a larger initial retention of stellar mass BHs the evolution depends on the timescale of their subsequent dynamical ejection. An early dynamical ejection of BHs results in a long-term evolution of the three parameters similar to that found in systems with no initial BH retention. Conversely, in the model that retains a large number of BHs for extended time (slow dynamical ejection of BHs), the system is characterized by a less concentrated structure and by the lack of significant temporal evolution of the three parameters. The smaller values of the three parameters found in this case might be used to indirectly infer the possible presence of BHs in the cluster.