Velocity-induced collapses of stable neutron stars

TL;DR

This paper investigates how adding inward velocity to stable neutron stars can induce collapse into black holes within General Relativity, revealing conditions for black hole formation, mass ejection, and scaling relations similar to critical collapse phenomena.

Contribution

It demonstrates velocity-induced collapse of neutron stars into black holes, including mass thresholds, matter ejection, and the application of Choptuik's scaling law to this process.

Findings

Neutron stars can collapse to black holes with sufficient inward velocity.

Part of the matter can be ejected during collapse, resulting in small black holes.

The mass scaling relation follows Choptuik's law for critical collapse.

Abstract

The collapse of spherical neutron stars is studied in General Relativity. The initial state is a stable neutron star to which an inward radial kinetic energy has been added through some velocity profile. For two different equations of state and two different shapes of velocity profiles, it is found that neutron stars can collapse to black holes for high enough inward velocities, provided that their masses are higher than some minimal value, depending on the equation of state. For a polytropic equation of state of the form $p=K\rho^\gamma $, with $\gamma = 2$ it is found to be $1.16 (\frac{K}{0.1})^{0.5} \msol$, whereas for a more realistic one, it reads $0.36 \msol $. In some cases of collapse forming a black hole, part of the matter composing the initial neutron star can be ejected through a shock, leaving only a fraction of the initial mass to form a black hole. Therefore, black holes of very small masses can be formed and, in particular, the mass scaling relation, as a function of initial velocity, takes the form discovered by Choptuik for critical collapses.