Maximal mass of the neutron star with a deconfined quark core

Abstract

The nature of equation of state for the matter in the neutron star plays an important role in determining its maximal mass. In addition, it must comply with the condition of causality. Noting that the central density of a maximally massive neutron star is well above the nuclear saturation density, a deconfined quark core in the central region is motivated in this paper. We analyze this scenario by employing the MIT bag model to represent the core region and one of the unified equations of state for the region outside the core. Such a combination is found to solve the problem of causality violation. In each case of the combined equations of state, the radial profile of $\rho r^2$ displays a peak and the dominant contribution to the total mass of the star comes from the region around the peak value of $\rho r^2$, whereas the contribution is small from the regions near the center and the surface. This peak occurs in the region of hadronic matter for the combinations considered in this paper. Importantly, we find that the position of the peak in $\rho r^2$ is well-correlated with the maximal mass—the highest value of 1.98 $M_\odot$ obtains for the case with the peak occurring farthest from the center. This gravitational threshold being obtained for a non-rotating neutron star, we expect the threshold to lie well above 2 $M_\odot$ for a rapidly rotating neutron star, that may explain the existence of massive pulsars from recent astronomical observations.