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

¹ Institut d’Astronomie et d’Astrophysique, CP226, Université Libre de Bruxelles, 1050 Brussels, Belgium
e-mail: nchamel@ulb.ac.be
² Dépt. de Physique, Université de Montréal, Montréal ( Québec), H3C 3J7, Canada

Received: 21 December 2012
Accepted: 18 February 2013

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. 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/√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.