# Isotropic-Nematic Phase Transitions in Gravitational Systems

## TL;DR

This paper models dense stellar systems with a central black hole as liquid crystal-like phases, revealing phase transitions between ordered disk-like and disordered isotropic states based on angular momentum.

## Contribution

It introduces a novel analogy between gravitational systems and liquid crystal phases, demonstrating phase transitions driven by angular momentum in such systems.

## Key findings

- Existence of ordered and disordered phases in stellar systems
- Identification of a first-order phase transition at a critical angular momentum
- Discovery of metastable states with mutually inclined disks

## Abstract

We examine dense self-gravitating stellar systems dominated by a central potential, such as nuclear star clusters hosting a central supermassive black hole. Different dynamical properties of these systems evolve on vastly different timescales. In particular, the orbital-plane orientations are typically driven into internal thermodynamic equilibrium by vector resonant relaxation before the orbital eccentricities or semimajor axes relax. We show that the statistical mechanics of such systems exhibit a striking resemblance to liquid crystals, with analogous ordered-nematic and disordered-isotropic phases. The ordered phase consists of bodies orbiting in a disk in both directions, with the disk thickness depending on temperature, while the disordered phase corresponds to a nearly isotropic distribution of the orbit normals. We show that below a critical value of the total angular momentum, the system undergoes a first-order phase transition between the ordered and disordered phases. At the critical point the phase transition becomes second-order while for higher angular momenta there is a smooth crossover. We also find metastable equilibria containing two identical disks with mutual inclinations between $90^{\circ}$ and $180^\circ$.