THE INNER BOUNDARY OF A NEUTRON-STAR CRUST

The phase boundary between the liquid matter of the interior of a neutron star and the solid matter that comprises the crust is studied. The matter is assumed to consist of electrons and non-relativistic neutrons and protons in beta equilibrium. The characteristic used to identify the phase boundary is the onset of instability of the uniform matter against proton clustering, and it is examined for four interactions either used previously in astrophysical studies - Skyrme 1', SkM and FPS - or proposed here as an improvement - FPS21. The relationship of this important property of neutron-star matter to the neutron-matter character of each interaction is explored. Characteristic densities associated with proton drip and with the liquid-gas boundary of one- and two-fluid models of the matter are compared with the instability density. Properties of some other effective interactions, relativistic and non-relativistic, are compared with those of the interactions discussed. The desirability of more calculations of neutron-matter properties, and of the utilization of those that do exist when formulating dense-matter equations of state, is emphasized. For the improved interaction FPS21, the crust fraction of the moment of inertia of a neutron star is estimated.