Global and crustal properties of neutron stars

Abstract

We investigate how the treatment of the neutron star crust affects both the global properties of the star and the properties of the crust itself. Three approaches are compared: a direct connection between a fixed crust and the core equation of state (EOS), a smooth crossover between them, and a unified EOS in which the crust is computed within a compressible liquid-drop model with the same nuclear empirical parameters of the relativistic mean-field model used for the core. For each approach, we consider four values of the symmetry energy slope, $44 \le L \le 92$ MeV. For a $1.4\,M_\odot$ star, some global properties are weakly to moderately affected: the radius and the moment of inertia agree within 1\% among the three approaches. On the other hand, the tidal deformability obtained with the direct connection is up to 21\% lower than the unified one for $L = 92$ MeV. A much larger contrast comes from the crustal properties. Relative to the unified EOS, the direct connection yields a crust that is 15--36\% thinner, 6--12 times less massive, and that carries a fraction of the total moment of inertia 5--8 times smaller (0.3--1.1\% instead of 2.8--6.2\%). The differences between crust treatments are thus comparable to or larger than those arising from variations in $L$ for crustal properties, while they remain subleading for global ones.