Scalarized neutron stars with a highly relativistic core in scalar-tensor gravity

TL;DR

This paper investigates highly relativistic neutron stars in scalar-tensor gravity, revealing multiple scalarized solutions and their dependence on the coupling function, with implications for pulsar and gravitational-wave observations.

Contribution

It uncovers the origin of multiple scalarized solutions in dense neutron stars and compares different scalar-tensor models based on their coupling functions.

Findings

Multiple scalarized solutions exist for fixed central density.

The multi-branch structure depends on the sign of the quadratic coupling coefficient.

Differences between models are due to whether the effective coupling function is bounded.

Abstract

Compact stars in scalar-tensor (ST) gravity have been extensively investigated, but relatively few studies have focused on highly relativistic neutron stars (NSs) with an extremely dense core region where the trace of the energy-momentum tensor reverses its sign. In this regime, we identify the origin of the phenomenon where {\it multiple} scalarized solutions exist for a {\it fixed} central density, arising from the oscillatory profile of the scalar field inside the star. This origin further indicates that the multi-branch structure emerges for both negative and positive $\beta$, the quadratic-term coefficient in the effective coupling function between the scalar field and conventional matter in the Einstein frame. By comparing the Damour--Esposito-Far\`ese and Mendes-Ortiz models of the ST gravity, we demonstrate that their distinct scalarization behaviors stem from whether the effective coupling function is bounded. We also compute for scalarized NSs with a highly relativistic dense core in ST theories the moment of inertia and tidal deformability that are relevant to pulsar-timing and gravitational-wave experiments.