Metastability of hadronic compact stars

Abstract

Pure hadronic compact stars, above a threshold value of their gravitational mass (central pressure), are metastable to the conversion to quark stars (hybrid or strange stars). In this paper, we present a systematic study of the metastability of pure hadronic compact stars using different relativistic models for the equation of state. In particular, we compare results for the quark-meson coupling model with those for the Glendenning-Moszkowski parametrization of the nonlinear Walecka model. For the quark-meson coupling model, we find large values (${M}_{\mathrm{cr}}=1.6--1.9{M}_{\ensuremath{\bigodot}}$) for the critical mass of the hadronic star sequence and we find that the formation of a quark star is only possible with a soft quark matter equation of state. For the Glendenning-Moszkowski parametrization of the nonlinear Walecka model, we explore the effect of different hyperon couplings on the critical mass and on the stellar conversion energy. We find that increasing the value of the hyperon coupling constants shifts the bulk transition point for quark deconfinement to higher densities, increases the stellar metastability threshold mass and the value of the critical mass, and thus makes the formation of quark stars less likely. For the largest values of the hyperon couplings we find a critical mass which may be as high as $1.9--2.1{M}_{\ensuremath{\bigodot}}$. These stellar configurations, which contain a large central hyperon fraction (${f}_{Y,\mathrm{cr}}\ensuremath{\sim}30%$), would be able to describe highly massive compact stars, such as the one associated with the millisecond pulsar PSR $\mathrm{B}1516+02\mathrm{B}$ with a mass $M={1.94}_{\ensuremath{-}0.19}^{+0.17}{M}_{\ensuremath{\bigodot}}$.