Influence of the nuclear symmetry energy on the structure and composition of the outer crust

TL;DR

This paper investigates how the nuclear symmetry energy influences the composition and structure of neutron star outer crusts, highlighting the importance of the symmetry energy's density dependence through nuclear models.

Contribution

It extends previous nonrelativistic mean field calculations by analyzing the impact of symmetry energy on crust composition using various nuclear models.

Findings

Stiffer symmetry energies lead to more exotic isotopes in the crust.

The pressure of the electron gas is well known, but the symmetry energy's density dependence is less certain.

Different nuclear models predict varying crust compositions based on symmetry energy characteristics.

Abstract

We review and extend with nonrelativistic nuclear mean field calculations a previous study of the impact of the nuclear symmetry energy on the structure and composition of the outer crust of nonaccreting neutron stars. First, we develop a simple "toy model" to understand the most relevant quantities determining the structure and composition of the outer crust: the nuclear symmetry energy and the pressure of the electron gas. While the latter is a well determined quantity, the former ---specially its density dependence--- still lacks an accurate characterization. We thus focus on the influence of the nuclear symmetry energy on the crustal composition. For that, we employ different nuclear models that are accurate in the description of terrestrial nuclei. We show that those models with stiffer symmetry energies ---namely, those that generate thicker neutron skins in heavy nuclei and have smaller symmetry energies at subnormal nuclear densities--- generate more exotic isotopes in the stellar crust than their softer counterparts.