Outer-Crust Equations of State for Neutron Stars

TL;DR

This study investigates how different nuclear mass models influence the composition and properties of neutron-star outer crusts, finding consistent predictions for key observables despite variations in nuclide sequences.

Contribution

It constructs and compares four outer-crust equations of state based on advanced nuclear models, highlighting their impact on neutron-star structure calculations.

Findings

Variations in nuclide sequences and neutron-drip density due to different mass models.

Thermodynamic properties and minimum-mass neutron-star configurations are consistent across models.

Outer-crust equations of state provide reliable inputs for neutron-star modeling despite model differences.

Abstract

The equation of state of the neutron-star outer crust is sensitive to nuclear mass predictions and provides a direct connection to properties of nuclei throughout the nuclide map, including those beyond experimental reach. We quantify the impact of contemporary nuclear mass models on the composition and thermodynamic properties of the outer crust, and assess the consequences for crust-dominated neutron-star configurations near the minimum-mass limit. We constructed four outer-crust equations of state based on the relativistic energy density functional and machine-learning mass model tables. The equilibrium composition of cold catalyzed matter in $\beta$-equilibrium was obtained by minimizing the Gibbs free energy per baryon, and the resulting equations of state were implemented in neutron-star structure calculations. The different mass inputs lead to variations in the equilibrium nuclide sequence, distinct last bound nuclei, and moderate shifts in the neutron-drip density. In contrast, the associated thermodynamic properties, as well as the minimum-mass neutron-star configurations, remain closely aligned across the four outer-crust equations of state. The model dependence of the outer crust is primarily reflected in the detailed nuclide composition and in the precise location of neutron drip. Nevertheless, the considered outer-crust equations of state yield closely consistent predictions for the relevant neutron-star observables, providing a reliable input for stellar modelling.