Scalar field induced oscillations of neutron stars and gravitational collapse

TL;DR

This paper investigates how massless scalar fields interact with neutron stars, causing oscillations or collapse, using advanced numerical simulations within general relativity to analyze energy transfer and gravitational signals.

Contribution

It introduces a fully dynamic, spherical symmetry simulation framework to study scalar field effects on neutron stars, revealing conditions for oscillation or collapse.

Findings

Scalar fields induce neutron star oscillations or collapse depending on compactness.

The simulations accurately extract gravitational radiation signals at null infinity.

Neutron stars exhibit quasi-normal mode oscillations before tail decay.

Abstract

We study the interaction of massless scalar fields with self-gravitating neutron stars by means of fully dynamic numerical simulations of the Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to spherical symmetry and the neutron stars are approximated by relativistic polytropes. Studying the nonlinear dynamics of isolated neutron stars is very effectively performed within the characteristic formulation of general relativity, in which the spacetime is foliated by a family of outgoing light cones. We are able to compactify the entire spacetime on a computational grid and simultaneously impose natural radiative boundary conditions and extract accurate radiative signals. We study the transfer of energy from the scalar field to the fluid star. We find, in particular, that depending on the compactness of the neutron star model, the scalar wave forces the neutron star either to oscillate in its radial modes of pulsation or to undergo gravitational collapse to a black hole on a dynamical timescale. The radiative signal, read off at future null infinity, shows quasi-normal oscillations before the setting of a late time power-law tail.