Average speed of sound in neutron stars

TL;DR

This paper explores how the thermodynamic properties, especially the speed of sound, relate to the dense matter equation of state in neutron stars, revealing that the speed of sound likely exceeds the conformal limit under certain conditions.

Contribution

It demonstrates that the vanishing of the trace anomaly in neutron star cores implies a superconformal speed of sound and introduces a new perspective on the stability of hybrid neutron stars.

Findings

Speed of sound likely exceeds the conformal value in neutron star cores.

Vanishing trace anomaly leads to nonmonotonic speed of sound behavior.

Local peak of the speed of sound is induced by positive trace anomaly.

Abstract

The structure of the dense-matter equation of state is essential for the phenomenology of neutron stars. In this work, I relate the thermodynamic properties to the average speed of sound in the interior of a star. I study the consequences of the vanishing of the trace anomaly. In particular, I show that if the trace anomaly vanishes in the centers of maximally massive neutron stars, the speed of sound likely exceeds its conformal value and exhibits nonmonotonic behavior. I also find that the additional assumption of positive definiteness of the trace anomaly naturally induces a local peak of the speed of sound at densities realized in the cores of neutron stars. I also reanalyze the stability condition for hybrid neutron stars. Possible implications for the dense matter equation of state and the phenomenology of neutron stars are also discussed.