Entropy Production in Collisionless Systems. II. Arbitrary Phase-Space Occupation Numbers

TL;DR

This paper compares two thermodynamic methods for determining equilibrium states of self-gravitating systems, extending previous analyses by avoiding Stirling approximation and applying to both collisionless and collisional models, finding that entropy production extremization yields maximum entropy configurations.

Contribution

It introduces a Stirling-approximation-free analysis of entropy maximization and production extremization for self-gravitating systems, unifying collisionless and collisional models under a common framework.

Findings

Entropy extremization leads to maximum entropy states.

Both Lynden-Bell and Maxwell-Boltzmann approaches produce the same equilibria.

The methods are valid for systems with finite mass and arbitrary phase-space occupation.

Abstract

We present an analysis of two thermodynamic techniques for determining equilibria of self-gravitating systems. One is the Lynden-Bell entropy maximization analysis that introduced violent relaxation. Since we do not use the Stirling approximation which is invalid at small occupation numbers, our systems have finite mass, unlike Lynden-Bell's isothermal spheres. (Instead of Stirling, we utilize a very accurate smooth approximation for $\ln{x!}$.) The second analysis extends entropy production extremization to self-gravitating systems, also without the use of the Stirling approximation. In addition to the Lynden-Bell (LB) statistical family characterized by the exclusion principle in phase-space, and designed to treat collisionless systems, we also apply the two approaches to the Maxwell-Boltzmann (MB) families, which have no exclusion principle and hence represent collisional systems. We implicitly assume that all of the phase-space is equally accessible. We derive entropy production expressions for both families, and give the extremum conditions for entropy production. Surprisingly, our analysis indicates that extremizing entropy production rate results in systems that have maximum entropy, in both LB and MB statistics. In other words, both thermodynamic approaches lead to the same equilibrium structures.