Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars

TL;DR

This paper investigates the stability and collapse of anisotropic neutron stars using various anisotropy models, analyzing their oscillations, and modeling their non-adiabatic gravitational collapse leading to black hole formation.

Contribution

It introduces a dynamical model for non-adiabatic collapse of anisotropic neutron stars and examines stability criteria beyond standard methods.

Findings

Standard stability criterion $dM/d ho_c >0$ is not always reliable for anisotropic stars.

Constraints on anisotropy parameters are derived from maximum mass observations.

The model describes the evolution of physical quantities during collapse.

Abstract

In this work we construct families of anisotropic neutron stars for an equation of state compatible with the constraints of the gravitational-wave event GW170817 and for four anisotropy ansatze. Such stars are subjected to a radial perturbation in order to study their stability against radial oscillations and we develop a dynamical model to describe the non-adiabatic gravitational collapse of the unstable anisotropic configurations whose ultimate fate is the formation of a black hole. We find that the standard criterion for radial stability $dM/d\rho_c >0$ is not always compatible with the calculation of the oscillation frequencies for some anisotropy ansatze, and each anisotropy parameter is constrained taking into account the recent restriction of maximum mass of neutron stars. We further generalize the TOV equations within a non-adiabatic context and we investigate the dynamical behaviour of the equation of state, heat flux, anisotropy factor and mass function as an unstable anisotropic star collapses. After obtaining the evolution equations we recover, as a static limit, the background equations.