New {\em ab initio} constrained extended Skyrme equations of state for simulations of neutron stars, supernovae and binary mergers: I. Subsaturation density domain

In numerical simulations of core-collapse supernova and binary neutron stars mergers, information about the energetics and composition of matter is implemented via external tables covering the huge ranges of thermodynamic conditions explored during the astrophysical evolution. More than 120 general-purpose equation of state (EOS) tables have been contributed so far. Unfortunately, not all of them comply with current constraints from theoretical and experimental nuclear physics and astrophysical observations of neutron stars. Systematic investigations of the role that dense matter properties play in the evolution of these astrophysical phenomena require that more EOS tables are provided. We build a set of general-purpose EOS tables. At zero temperature, they comply with all currently accepted constraints, including {\em ab initio} chiral effective field theory calculations of pure neutron matter. This set is designed to explore a wide variety of the behaviors of the effective masses as functions of density, which is reflected into a wide range of thermal behaviors. We employ Brussels extended Skyrme interactions generated by means of Bayesian inference techniques. An extended nuclear statistical equilibrium model is developed for modeling sub-saturated inhomogeneous nuclear matter (NM). Here, we study the properties of sub-saturated inhomogeneous NM over wide ranges of density, temperature, and proton fraction. We analyze in detail the mechanisms of transition to homogeneous matter and estimate the transition density. Our key results include a thick layer of neutron rich isotopes of He or H in the inner crusts of neo-neutron stars, significant abundance of exotic isotopes of H and He in warm and neutron-rich matter and a detailed study of the thermodynamic stability of cold stellar matter. The EOS tables are publicly available in the \textsc{CompOSE} online database.