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

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.

Towards generating a new supernova equation of state: A systematic analysis of cold hybrid stars

“…(18) reveals that more than a single phase transition can happen. Multiple phase transitions were also found for other hybrid EOSs; see, e.g., [19,36,37,55]. Figure 7 shows an example where three phase transitions occur.…”

“…The problem of reconfinement is known in the literature (e.g. [35][36][37]), but in the parameter scans of Alford et al and subsequent works it was not addressed. If one does not consider reconfinement, and by doing so ignores thermodynamic stability in a strict sense, this leads to extremely high neutron star masses of over 3 M ⊙ at low transition pressures.…”

Neutron-star matter within the energy-density functional theory and neutron-star structure

“…On the other hand, as discussed in Ref. [54], considering the occurrence of a transition to an exotic phase in NS cores could increase the maximum mass, thus making our soft EoS based on the BSk19 EDF still compatible with the existence of two solar mass NSs.…”

“…The possibility of a transition to non-hadronic matter using the BSk19, BSk20, and BSk21 functionals and the impact on the NS structure have been discussed in Ref. [54].…”

Analytical solutions of equation for the order parameter of dense superfluid neutron matter with anisotropic spin-triplet p-wave pairing at finite temperatures

“…Here we apply generalized BSk21 and BSk24 Skyrme forces [47,48] which lead to sufficiently stiff equations of state of dense pure neutron matter (NM) and are consistent (see [45][46][47] [49] and PSR J0348-0432 [50]. Note that selected here BSk21 and BSk24 are most likely the best parametrizations among other generalized parametrizations of the Skyrme forces (see Conclusions in [47]) which are sufficiently accurate in calculation of neutron effective mass (which is strongly density dependent).…”