Differentially rotating neutron stars in scalar-tensor theories of gravity

TL;DR

This paper introduces numerical models of differentially rotating neutron stars within scalar-tensor gravity theories, revealing significant deviations from general relativity in maximum mass, angular momentum, and stability properties.

Contribution

First numerical models of differentially rotating neutron stars in scalar-tensor theories, showing enhanced maximum mass and altered rotational characteristics compared to GR.

Findings

Scalarized stars can reach larger masses and angular momenta.

Scalar fields increase the axis ratio needed for given angular momentum.

Presence of scalar fields affects stability and potential remnants in mergers.

Abstract

We present the first numerical models of differentially rotating stars in alternative theories of gravity. We chose a particular class of scalar-tensor theories of gravity that is indistinguishable from GR in the weak field regime but can lead to significant deviations when strong fields are considered. We show that the maximum mass that a differentially rotating neutron star can sustain increases significantly for scalarized solutions and such stars can reach larger angular momenta. In addition, the presence of a nontrivial scalar field has the effect of increasing the required axis ratio for reaching a given value of angular momentum, when compared to a corresponding model of same rest mass in general relativity. We find that the scalar field also makes rapidly rotating models less quasi-toroidal than their general-relativistic counterparts. For large values of the angular momentum and values of the coupling parameter that are in agreement with the observations, we find a second turning point for scalarized models along constant angular momentum sequences, which could have interesting implications for the stability of remnants created in a binary neutron star merger.