Compactness of neutron stars and Tolman VII solutions in scalar-tensor gravity

TL;DR

This paper investigates the maximum compactness of neutron stars within scalar-tensor gravity, comparing it to general relativity, and finds that scalar-tensor theories limit the formation of ultra-compact stars.

Contribution

It provides a systematic analysis of neutron star compactness in scalar-tensor gravity using Tolman VII solutions, highlighting differences from general relativity.

Findings

Maximum neutron star compactness is lower in scalar-tensor gravity than in general relativity.

Ultra-compact stars cannot be scalarized in scalar-tensor theories, even with uniform density.

Scalar-tensor coupling constraints restrict neutron star properties.

Abstract

We systematically examine the compactness of neutron stars as Tolman VII solutions in scalar-tensor theory of gravity. As a result, when the coupling constant is confined to values provided by astronomical observations we show that the maximum compactness of neutron stars in general relativity is higher than that in scalar-tensor gravity. In addition, we show that although ultra-compact stars, with radius smaller than the Regge-Wheeler potential peak, can exist in general relativity (e.g., Tolman VII solution), their scalarized counterparts cannot {be constructed} even in the limiting case of uniform density stars.