New ab initio constrained extended Skyrme equations of state for simulations of neutron stars, supernovae, and binary mergers

I. Subsaturation density domain

National Institute for Physics and Nuclear Engineering (IFIN-HH), RO-077125 Bucharest, Romania

^⋆ Corresponding authors: araduta@nipne.ro; mikhail.beznogov@nipne.ro

Received: 30 April 2025
Accepted: 2 August 2025

Abstract

Context. In numerical simulations of core-collapse supernovae and mergers of binary neutron stars, information about the energetics and composition of matter is implemented via external tables covering the wide range of thermodynamic conditions explored during astrophysical evolution. More than 120 general purpose equation of state (EOS) tables have been contributed so far. Unfortunately, not all of them comply with the 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.

Aims. In this work, we build a set of general purpose EOS tables. At zero temperature, they comply with all currently accepted constraints, including ab initio chiral effective field theory calculations of pure neutron matter. This set is designed to explore the wide variety of behaviors of the effective masses as functions of density, which are reflected in a wide range of thermal behaviors. Methods. An extended nuclear statistical equilibrium model is developed for modeling subsaturated inhomogeneous nuclear matter (NM).

Results. We study the properties of subsaturated inhomogeneous NM over wide ranges of density, temperature, and proton fraction. We analyze 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, a 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 CompOSE online database.