The Kinematic Richness of Star Clusters I. Isolated Spherical Models with Primordial Anisotropy

TL;DR

This study uses N-body simulations to explore how primordial velocity anisotropy influences the core collapse timing in isolated star clusters, revealing a monotonic relationship between anisotropy and collapse time.

Contribution

It demonstrates the impact of initial velocity anisotropy on cluster evolution and introduces a fluid model to interpret these effects, advancing understanding of stellar system dynamics.

Findings

Tangential anisotropy delays core collapse

Radial anisotropy accelerates core collapse

Inner catastrophic collapse occurs in tangentially anisotropic models

Abstract

We investigate the dynamical evolution of isolated equal-mass star cluster models by means of direct N-body simulations, primarily focusing on the effects of the presence of primordial anisotropy in the velocity space. We found evidence of the existence of a monotonic relationship between the moment of core collapse and the amount and flavour of anisotropy in the stellar system. Specifically, equilibria characterised by the same initial structural properties (Plummer density profile) and with different degrees of tangentially-biased (radially-biased) anisotropy, reach core collapse earlier (later) than isotropic models. We interpret this result in light of an accelerated (delayed) phase of the early evolution of collisional stellar systems "anisotropic-response"), which we have characterised both in terms of the evolution of the velocity moments and of a fluid model of two-body relaxation. For the case of the most tangentially anisotropic model the initial phase of evolution involves a catastrophic collapse of the inner part of the system which continues until an isotropic velocity distribution is reached. This study represents a first step towards a comprehensive investigation of the role played by kinematic richness in the long-term dynamical evolution of collisional systems.