Dynamics of star clusters with tangentially anisotropic velocity distribution

TL;DR

This study uses N-body simulations to explore how different initial velocity anisotropies and external tidal fields influence the dynamical evolution, relaxation, and energy equipartition in star clusters, revealing significant effects of tangential anisotropy.

Contribution

It provides a comprehensive analysis of how tangential and radial velocity anisotropies affect star cluster evolution, including relaxation rates and anisotropy profiles, expanding theoretical understanding.

Findings

Tangential anisotropy accelerates relaxation and core collapse.

Outer regions develop tangential or radial anisotropy depending on tidal filling.

Energy equipartition is inverted in outer regions, influenced by initial anisotropy.

Abstract

Recent high-precision observations with HST and Gaia enabled new investigations of the internal kinematics of star clusters (SCs) and the dependence of kinematic properties on the stellar mass. These studies raised new questions about the dynamical evolution of self-gravitating stellar systems. We aim to develop a more complete theoretical understanding of how various kinematical properties of stars affect the global dynamical development of their host SCs. We perform N-body simulations of SCs with isotropic, radially anisotropic and tangentially anisotropic initial velocity distributions. We also study the effect of an external Galactic tidal field. We find three main results. First, compared to the conventional, isotropic case, the relaxation processes are accelerated in the tangentially anisotropic models and, in agreement with our previous investigations, slower in the radially anisotropic ones. This leads to, e.g., more rapid mass segregation in the central regions of the tangential models or their earlier core collapse. Second, although all SCs become isotropic in the inner regions after several relaxation times, we observe differences in the anisotropy profile evolution in the outer cluster regions - all tidally filling models gain tangential anisotropy there while the underfilling models become radially anisotropic. Third, we observe different rates of evolution towards energy equipartition (EEP). While all SCs evolve towards EEP in their inner regions (regardless of the filling factor), the outer regions of the tangentially anisotropic and isotropic models are evolving to an "inverted" EEP (i.e., the high-mass stars having higher velocity dispersion than the low-mass ones). The extent (both spatial and temporal) of this inversion can be attributed to the initial velocity anisotropy - it grows with increasing tangential anisotropy and decreases as the radial anisotropy rises.