Rigidly rotating perfect fluid stars in $2+1$ dimensions

TL;DR

This paper classifies all rigidly rotating perfect fluid star solutions in 2+1 dimensions with negative cosmological constant, detailing their properties, parameter space, and causal structure, and relating them to BTZ black hole solutions.

Contribution

It provides a comprehensive analysis of rotating perfect fluid stars in 2+1 dimensions, including solution classification, parameter space characterization, and causal structure, extending previous work by Cataldo.

Findings

Existence of solutions in all three BTZ classes: black hole, point particle, overspinning.

Unique solutions for each (tilde J, M) in specific parameter regions.

All stars have anti-de Sitter causal structure, without horizons or closed timelike curves.

Abstract

Cataldo has found all rigidly rotating self-gravitating perfect fluid solutions in 2+1 dimensions with a negative cosmological constant $\Lambda$, for a density that is specified a priori as a function of a certain radial coordinate. We rewrite these solutions in standard polar-radial coordinates, for an arbitrary barotropic equation of state $p(\rho)$. For any given equation of state, we find the two-parameter family of solutions with a regular centre and finite total mass $M$ and angular momentum $J$ (rigidly rotating stars). For analytic equations of state, the solution is analytic except at the surface, but including at the centre. Defining the dimensionless spin $\tilde J:=\sqrt{-\Lambda}\,J$, there is precisely one solution for each $(\tilde J,M)$ in the region $|\tilde J|-1<M<|\tilde J|$, which consists of parts of the point particle region $M<-|\tilde J|$ and overspinning regions $|\tilde J|>|M|$. In an adjacent compact part of the black hole region $|\tilde J|<M$ (whose extent depends on the equation of state), there are precisely two solutions for each $(\tilde J,M)$. Hence exterior solutions exist in all three classes of BTZ solution (black hole, point particle and overspinning), but not all possible values of $(\tilde J,M)$ can be realised as stars. Regardless of the values of $\tilde J$ and $M$, the causal structure of all stars for all equations of state is that of anti-de Sitter space, without horizons or closed timelike curves.