Phase transitions in dense matter and the maximum mass of neutron stars

TL;DR

This paper investigates how phase transitions to exotic matter in neutron star cores affect the maximum mass of neutron stars, considering various equations of state and the implications of recent observations.

Contribution

It introduces a comprehensive analysis of the impact of first-order phase transitions and exotic cores on neutron star maximum mass, including the effects of quark matter and causality constraints.

Findings

Exotic cores can increase maximum neutron star mass despite softening of the EoS.

Quark matter reduces maximum mass if sound speed is limited by c/√3.

Support for PSR J1614-2230 is possible with a soft nucleonic EoS compatible with heavy-ion data.

Abstract

The recent precise measurement of the mass of pulsar PSR J1614$-$2230, as well as observational indications of even more massive neutron stars, has revived the question of the composition of matter at the high densities prevailing inside neutron-star cores. We study the impact on the maximum possible neutron-star mass of an "exotic" core consisting of non-nucleonic matter. For this purpose, we study the occurrence of a first-order phase transition in nucleonic matter. Given the current lack of knowledge of non-nucleonic matter, we consider the stiffest possible equation of state subject only to the constraints of causality and thermodynamic stability. The case of a hadron-quark phase transition is discussed separately. The purely nucleonic matter is described using a set of unified equations of state that have been recently developed to permit a consistent treatment of both homogeneous and inhomogeneous phases. We then compute the mass-radius relation of cold nonaccreting neutron stars with and without exotic cores from the Tolman-Oppenheimer-Volkoff equations. We find that even if there is a significant softening of the equation of state associated with the actual transition to an exotic phase, there can still be a stiffening at higher densities closer to the center of the star that is sufficient to increase the maximum possible mass. However with quarks the maximum neutron-star mass is always reduced by assuming that the sound speed is limited by $c/\sqrt{3}$ as suggested by QCD calculations. In particular, by invoking such a phase transition, it becomes possible to support PSR J1614$-$2230 with a nucleonic equation of state that is soft enough to be compatible with the kaon and pion production in heavy-ion collisions.