Collapse of differentially rotating neutron stars and cosmic censorship

TL;DR

This study investigates the collapse dynamics of differentially rotating neutron stars, revealing that sub-Kerr stars can collapse into black holes with gravitational wave emission, while supra-Kerr stars do not naturally collapse into naked singularities, supporting cosmic censorship.

Contribution

It provides new insights into the collapse behavior of differentially rotating neutron stars with varying angular momentum, highlighting stability conditions and implications for cosmic censorship.

Findings

Sub-Kerr models can collapse promptly into black holes.

Supra-Kerr models are stable and do not naturally collapse.

Collapse dynamics and gravitational waves resemble uniform rotation cases.

Abstract

We present new results on the dynamics and gravitational-wave emission from the collapse of differentially rotating neutron stars. We have considered a number of polytropic stellar models having different values of the dimensionless angular momentum J/M^2, where J and M are the asymptotic angular momentum and mass of the star, respectively. For neutron stars with J/M^2<1, i.e., "sub-Kerr" models, we were able to find models that are dynamically unstable and that collapse promptly to a rotating black hole. Both the dynamics of the collapse and the consequent emission of gravitational waves resemble the one seen for uniformly rotating stars, although with an overall decrease in the efficiency of gravitational-wave emission. For stellar models with J/M^2>1, i.e., "supra-Kerr" models, on the other hand, we were not able to find models that are dynamically unstable and all of the computed supra-Kerr models were found to be far from the stability threshold. For these models a gravitational collapse is possible only after a very severe and artificial reduction of the pressure, which then leads to a torus developing nonaxisymmetric instabilities and eventually contracting to a stable axisymmetric stellar configuration. While this does not exclude the possibility that a naked singularity can be produced by the collapse of a differentially rotating star, it also suggests that cosmic censorship is not violated and that generic conditions for a supra-Kerr progenitor do not lead to a naked singularity.