Average speed of sound in neutron stars

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.