Extended quark mean-field model for neutron stars

Abstract

We extend the quark mean-field (QMF) model to strangeness freedom to study the properties of hyperons ($\ensuremath{\Lambda},\ensuremath{\Sigma},\ensuremath{\Xi}$) in infinite baryon matter and neutron star properties. The baryon-scalar meson couplings in the QMF model are determined self-consistently from the quark level, where the quark confinement is taken into account in terms of a scalar-vector harmonic oscillator potential. The strength of such confinement potential for $u,d$ quarks is constrained by the properties of finite nuclei, while that for an $s$ quark is limited by the properties of nuclei with a $\ensuremath{\Lambda}$ hyperon. These two strengths are not the same, which represents the SU(3) symmetry breaking effectively in the QMF model. Also, we use an enhanced $\ensuremath{\Sigma}$ coupling with the vector meson, and both $\ensuremath{\Sigma}$ and $\ensuremath{\Xi}$ hyperon potentials can be properly described in the model. The effects of the SU(3) symmetry breaking on the neutron star structures are then studied. We find that the SU(3) breaking shifts the hyperon onset density earlier and makes hyperons more abundant in the star, in comparison with the results of the SU(3) symmetry case. However, it has little effect on the star's maximum mass. The maximum masses are found to be $1.62{M}_{\ensuremath{\bigodot}}$ with hyperons and $1.88{M}_{\ensuremath{\bigodot}}$ without hyperons. The present neutron star model is shown to have limitations in explaining the recently measured heavy pulsars around $2{M}_{\ensuremath{\bigodot}}$.