Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. XII: Stiffness and stability of neutron-star matter

TL;DR

This paper introduces three new Skyrme-Hartree-Fock-Bogoliubov mass models with density-dependent forces, improving stability and fit to nuclear data, and enabling comprehensive neutron star studies.

Contribution

The paper develops three novel HFB mass models with unconventional Skyrme forces fitted to various neutron matter equations of state, eliminating instabilities and enhancing neutron star modeling.

Findings

Models fit nuclear mass data with 0.58 MeV rms deviation

All models eliminate nuclear matter instabilities

Models are suitable for neutron star crust and core studies

Abstract

We construct three new Hartree-Fock-Bogoliubov (HFB) mass models, labeled HFB-19, HFB-20, and HFB-21, with unconventional Skyrme forces containing $t_4$ and $t_5$ terms, i.e., density-dependent generalizations of the usual $t_1$ and $t_2$ terms, respectively. The new forces underlying these models are fitted respectively to three different realistic equations of state of neutron matter for which the density dependence of the symmetry energy ranges from the very soft to the very stiff, reflecting thereby our present lack of complete knowledge of the high-density behavior of nuclear matter. All unphysical instabilities of nuclear matter, including the transition to a polarized state in neutron-star matter, are eliminated with the new forces. At the same time the new models fit essentially all the available mass data with rms deviations of 0.58 MeV and give the same high quality fits to measured charge radii that we obtained in earlier models with conventional Skyrme forces. Being constrained by neutron matter, these new mass models, which all give similar extrapolations out to the neutron drip line, are highly appropriate for studies of the $r$-process and the outer crust of neutron stars. Moreover, the underlying forces, labeled BSk19, BSk20 and BSk21, respectively, are well adapted to the study of the inner crust and core of neutron stars. The new family of Skyrme forces thus opens the way to a unified description of all regions of neutron stars.