Spherical neutron star collapse toward a black hole in tensor-scalar theory of gravity

TL;DR

This paper models the gravitational collapse of neutron stars into black holes within tensor-scalar gravity, revealing spontaneous scalarization effects that influence gravitational wave signals, with implications for future detection constraints.

Contribution

It presents the first complete tensor-scalar and hydrodynamic simulation of neutron star collapse, highlighting spontaneous scalarization and its impact on gravitational wave emission.

Findings

Scalar gravitational wave amplitude increases dramatically above a critical coupling parameter.

Spontaneous scalarization occurs, not present in Brans-Dicke theory.

Collapse signals are unlikely detectable beyond a few hundred kiloparsecs.

Abstract

Complete tensor-scalar and hydrodynamic equations are presented and integrated, for a self-gravitating perfect fluid. The initial conditions describe unstable-equilibrium neutron star configuration, with a polytropic equation of state. They are necessary in order to follow the gravitational collapse (including full hydrodynamics) of this star toward a black hole and to study the resulting scalar gravitational wave. The amplitude of this wave, as well as the radiated energy dramatically increase above some critical value of the parameter of the coupling function, due to the spontaneous scalarization, an effect not present in Brans-Dicke theory. In most cases, the pressure of the collapsing fluid does not have a significant impact on the resulting signal. These kind of sources are not likely to be observed by future laser interferometric detectors (such as VIRGO or LIGO) of gravitational waves, if they are located at more than a few 100 kpc. However, spontaneous scalarization could be constrained if such a gravitational collapse is detected by its quadrupolar gravitational signal, since this latter is quite lower than the monopolar one.