Massive neutron stars with small radii in relativistic mean-field models optimized to nuclear ground states

Abstract: We present an equation of state (EoS) for neutron stars using the relativistic mean-field model with isoscalar- and isovector-meson mixing. Taking into account the results of the neutron skin thickness, $R_{\rm skin}$, of $^{208}$Pb reported by the PREX collaboration, the dimensionless tidal deformability of a canonical neutron star observed from GW170817, and a $2.6$ $M_{\odot}$ compact star implied by the secondary component of GW190814, a new effective interaction is constructed so as to reproduce the saturation condition of nuclear matter and the ground-state properties of finite, closed-shell nuclei. We find that the neutron star EoS exhibits the rapid stiffening around twice the nuclear saturation density, which is caused by the soft nuclear symmetry energy, $E_{\rm sym}$. It is also noticeable that the thick $R_{\rm skin}$ from the PREX-2 experiment can be achieved with the small slope parameter of $E_{\rm sym}$ stemming from the isoscalar-meson mixing. Thus, we speculate that the secondary component of GW190814 is the heaviest neutron star ever discovered.