A generalized Landau kinetic equation for weakly-coupled probability distribution of N-stars in dense star cluster

TL;DR

This paper develops a generalized Landau kinetic equation for dense star clusters, incorporating a correct cut-off process that influences relaxation times and avoids infinite-density issues, providing new insights into cluster evolution.

Contribution

It introduces a self-consistent truncated BBGKY hierarchy-based kinetic equation that accurately models the effect of total star number and cut-offs on cluster structure and relaxation.

Findings

Cut-off effects increase relaxation time by a few percent.

Proper cut-offs prevent infinite-density problems in core-collapse.

Total star number influences density profiles and mean-field acceleration.

Abstract

The secular evolution of a collisional star cluster of $N$-'point' stars have been conventionally discussed based on cumulative two-body relaxation process. The relaxation process requires a cut-off on the range of two-body encounter between stars in physical space and the relaxation time is characterized by Coulomb logarithm $\ln[N]$; the conventional cut-off on the encounter distance in the literature gives "dominant" effect. In addition, incorrect cut-offs exposed a mathematical "infinite-density" problem in the late stage of core-collapse. The present paper shows these are merely the results due to incorrect cut-off process. If one correctly constrains the cut-off on interaction range between stars based on truncated BBGKY hierarchy, one must introduce a self-consistent 'truncated' Newtonian mean-field (m.f.) acceleration of star. The present paper shows the effect of total number on the structure of finite star cluster (in which stars undergo only two-body encounters) for the first time after establishing a mathematical formulation of a generalized Landau kinetic equation that includes the cut-off effect on both of collision term and m.f. potential by employing a BBGKY hierarchy for truncated DF stars. The cut-off effect increases the typical relaxation time by a few of percentage, which means the effect of cut-off itself is "non-dominant" on the relaxation time. On the other hand, the cut-off on m.f. acceleration is necessary to avoid "infinite-density" problem at the center of the system; the effect of total number $N$ on density profile and m.f. acceleration are shown by applying the truncated-DF BBGKY hierarchy to a toy model (a quasi-static modified Hubble density profile) for a core-halo structure of a star cluster at the late stage of evolution.