Neutron stars in massive scalar-Gauss-Bonnet gravity: Spherical structure and time-independent perturbations

TL;DR

This paper studies neutron stars in a specific scalar-Gauss-Bonnet gravity theory, analyzing their structure and perturbations, and finds notable differences in tidal deformability compared to general relativity, impacting universal relations.

Contribution

It provides a detailed analysis of scalarized neutron stars with massive scalar fields, including their spherical structure and perturbative properties, extending understanding beyond GR.

Findings

Tidal deformability shows significant deviations from GR.

Universal I-Love relation breaks down in scalar-Gauss-Bonnet theory.

Mass, radius, and moment of inertia are moderately affected.

Abstract

The class of scalar-tensor theories with the scalar field coupling to the Gauss-Bonnet invariant has drawn great interest since solutions of spontaneous scalarization were found for black holes in these theories. We contribute to the existing literature a detailed study of the spontaneously scalarized neutron stars (NSs) in a typical theory where the coupling function of the scalar field takes the quadratic form and the scalar field is massive. The investigation here includes the spherical solutions of the NSs as well as their perturbative properties, namely the tidal deformability and the moment of inertia, treated in a unified and extendable way under the framework of spherical decomposition. We find that while the mass, the radius, and the moment of inertia of the spontaneously scalarized NSs show very moderate deviations from those of the NSs in general relativity (GR), the tidal deformability exhibits significant differences between the solutions in GR and the solutions of spontaneous scalarization for certain values of the parameters in the scalar-Gauss-Bonnet theory. As a result, the celebrated universal relation between the moment of inertia and the tidal deformability of neutron stars breaks down. With the mass and the tidal deformability of NSs attainable in the gravitational waves from binary NS mergers, the radius measurable using the X-ray satellites, and the moment of inertia accessible via the high-precision pulsar timing techniques, future multi-messenger observations can be contrasted with the theoretical results and provide us necessary information for building up theories beyond GR.