Magnetic neutron star equilibria with stratification and type-II superconductivity

TL;DR

This paper develops self-consistent two-fluid models of neutron stars with magnetic fields, incorporating stratification and superconductivity, to better understand their equilibrium configurations and magnetic properties.

Contribution

It introduces a nonlinear numerical approach for two-fluid neutron star models with stratification and superconductivity, advancing beyond the barotropic assumption.

Findings

Stratification enables stronger toroidal magnetic components.

Poloidal magnetic fields remain dominant in all models.

Quantitative magnetic ellipticities are provided for different cases.

Abstract

We construct two-fluid equilibrium configurations for neutron stars with magnetic fields, using a self-consistent and nonlinear numerical approach. The two-fluid approach - likely to be valid for large regions of all but the youngest NSs - provides us with a straightforward way to introduce stratification and allows for more realistic models than the ubiquitous barotropic assumption. In all our models the neutrons are modelled as a superfluid, whilst for the protons we consider two cases: one where they are a normal fluid and another where they form a type-II superconductor. We consider a variety of field configurations in the normal-proton case and purely toroidal fields in the superconducting case. We find that stratification allows for a stronger toroidal component in mixed-field configurations, though the poloidal component remains the largest in all our models. We provide quantitative results for magnetic ellipticities of NSs, both in the normal- and superconducting-proton cases.