How neutron star properties disfavor a nuclear chiral density wave

TL;DR

This study investigates how the presence of a chiral density wave in neutron star cores is disfavored when considering realistic astrophysical constraints, suggesting neutron stars likely have isotropic cores.

Contribution

The paper extends previous models by including charge neutrality, electroweak equilibrium, and neutron matter properties, showing these factors disfavor chiral density waves in neutron stars.

Findings

Chiral density waves are postponed to higher densities in neutron stars.

Such waves are only favored in parameter regions incompatible with observed star masses.

Astrophysical constraints imply neutron star cores are likely isotropic.

Abstract

Cold and dense matter may break rotational symmetry spontaneously and thus form an anisotropic phase in the interior of neutron stars. We consider the concrete example of an anisotropic chiral condensate in the form of a chiral density wave. Employing a nucleon-meson model and taking into account fermionic vacuum fluctuations, we improve and extend previous results by imposing the conditions of electric charge neutrality and electroweak equilibrium, by allowing for a more general form of the vector meson self-interactions, and by including properties of pure neutron matter into the fit of the model parameters. We find that the conditions inside neutron stars postpone the onset of the chiral density wave to larger densities compared to isospin-symmetric nuclear matter. While this still allows for the construction of stars with an anisotropic core, we find that the chiral density wave is energetically preferred only in a corner of the parameter space where matter is too soft to generate stars with realistic masses. Therefore, taking into account constraints from astrophysical data, our calculation predicts an isotropic neutron star core.