New insights into star cluster evolution towards energy equipartition

TL;DR

This study investigates how star clusters evolve towards energy equipartition, revealing that initial anisotropy influences the rate and nature of this process, with implications for understanding cluster dynamics and evolution.

Contribution

It demonstrates that initial velocity anisotropy affects the rate of energy equipartition and uncovers an inverted state in outer regions, advancing theoretical models of cluster evolution.

Findings

Radial anisotropy accelerates energy equipartition.

Outer regions can exhibit inverted energy equipartition.

Tangential velocity dispersion causes mass-dependent anomalies.

Abstract

We present the results of a study aimed at exploring the evolution towards energy equipartition in star cluster models with different initial degrees of anisotropy in the velocity distribution. Our study reveals a number of novel aspects of the cluster dynamics and shows that the rate of evolution towards energy equipartition (1) depends on the initial degree of radial velocity anisotropy -- it is more rapid for more radially anisotropic systems; and (2) differs for the radial and the tangential components of the velocity dispersion. (3) The outermost regions of the initially isotropic system evolve towards a state of `inverted' energy equipartition in which high-mass stars have a larger velocity dispersion than low-mass stars -- this inversion originates from the mass-dependence of the tangential velocity dispersion whereas the radial velocity dispersion shows no anomaly. Our results add new fundamental elements to the theoretical framework needed to interpret the wealth of recent and upcoming observational studies of stellar kinematics in globular clusters, and shed further light on the link between the clusters' internal kinematics, their formation and evolutionary history.