# An order-disorder phase transition in black-hole star clusters

## TL;DR

This paper reveals that star clusters around black holes can undergo a phase transition from spherical to lopsided configurations, significantly impacting transient event rates like tidal disruptions and gravitational waves.

## Contribution

It demonstrates a robust order-disorder phase transition in black-hole star clusters, challenging the common assumption of spherical symmetry in models.

## Key findings

- Clusters can transition to a lopsided, high-eccentricity state
- Ordered phase likely increases transient event rates
- Models need refinement to predict cluster phases accurately

## Abstract

The centers of most galaxies contain massive black holes surrounded by dense star clusters. The structure of these clusters determines the rate and properties of observable transient events, such as flares from tidally disrupted stars and gravitational-wave signals from stars spiraling into the black hole. Most estimates of these rates enforce spherical symmetry on the cluster. Here we show that, in the course of generic evolutionary processes, a star cluster surrounding a black hole can undergo a robust phase transition from a spherical thermal equilibrium to a lopsided equilibrium, in which most stars are on high-eccentricity orbits with aligned orientations. The rate of transient events is expected to be much higher in the ordered phase. Better models of cluster formation and evolution are needed to determine whether clusters should be found in the ordered or disordered phase.