Neutron Stars in Causal Scalar-Tensor Theories

TL;DR

This paper investigates how scalar-tensor theories with non-canonical kinetic terms affect neutron star structure, revealing multiple solution branches and critical densities where solutions cease to exist, with implications for astrophysical observations.

Contribution

It provides a numerical analysis of neutron stars in causal scalar-tensor theories with K-essence, highlighting the existence of multiple solution branches and critical densities.

Findings

Two distinct neutron star configurations for the same central density.

Existence of a critical central density beyond which solutions do not exist.

Predictions for mass-radius relations compared with observational data.

Abstract

We study static, spherically symmetric neutron stars in a class of scalar-tensor theories with non-canonical kinetic terms (K-essence) obeying all causality and hyperbolicity conditions. These models have non-trivial dynamics that lead to a type of anti-screening of the scalar. They lead to small corrections in the solar system due to a small coupling, but can lead to large corrections in regimes of high densities, especially neutron stars. We solve the modified Tolman-Oppenheimer-Volkoff equations numerically using realistic equations of state (SLy4, WFF1, MS1, MPA1). For a given central density, we find that two distinct configurations may exist, forming two separate branches of solutions. We find that above a certain critical central density solutions with the correct asymptotic behavior at spatial infinity cannot be obtained. We obtain precise predictions for the mass-radius relation for neutron stars for different values of the parameters in the model and we compare to data.