Structure of neutron stars in massive scalar-tensor gravity

TL;DR

This paper models neutron stars within massive scalar-tensor gravity theories, revealing how scalar fields influence their structure and stability, including novel configurations like scalar clouds around dense stars.

Contribution

It introduces a numerical method for modeling neutron stars in massive scalar-tensor theories and explores the effects of scalar coupling parameters on star structure and stability.

Findings

Identification of scalarized neutron-star branches

Discovery of 'gravitational atom' configurations with scalar clouds

Stability analysis indicating these configurations are unstable

Abstract

We compute families of spherically symmetric neutron-star models in two-derivative scalar-tensor theories of gravity with a massive scalar field. The numerical approach we present allows us to compute the resulting spacetimes out to infinite radius using a relaxation algorithm on a compactified grid. We discuss the structure of the weakly and strongly scalarized branches of neutron-star models thus obtained and their dependence on the linear and quadratic coupling parameters $\alpha_0$, $\beta_0$ between the scalar and tensor sectors of the theory, as well as the scalar mass $\mu$. For highly negative values of $\beta_0$, we encounter configurations resembling a "gravitational atom", consisting of a highly compact baryon star surrounded by a scalar cloud. A stability analysis based on binding-energ calculations suggests that these configurations are unstable and we expect them to migrate to models with radially decreasing baryon density {\it and} scalar field strength.