A superfluid perspective on neutron star dynamics

TL;DR

This paper reviews how superfluidity influences neutron star dynamics, including rotation, dissipation, and seismic activity, emphasizing the complex multi-fluid nature and the role of entrainment in their interior physics.

Contribution

It provides a comprehensive overview of superfluid effects on neutron star dynamics, highlighting recent insights into multi-fluid interactions and their astrophysical implications.

Findings

Superfluidity introduces new degrees of freedom in neutron star dynamics.

Quantised vortices support bulk rotation and cause mutual friction dissipation.

Entrainment significantly affects superfluid motion and star seismology.

Abstract

As mature neutron stars are cold (on the relevant temperature scale), one has to carefully consider the state of matter in their interior. The outer kilometer or so is expected to freeze to form an elastic crust of increasingly neutron-rich nuclei, coexisting with a superfluid neutron component, while the star's fluid core contains a mixed superfluid/superconductor. The dynamics of the star depend heavily on the parameters associated with the different phases. The presence of superfluidity brings new degrees of freedom -- in essence we are dealing with a complex multi-fluid system -- and additional features: Bulk rotation is supported by a dense array of quantised vortices, which introduce dissipation via mutual friction, and the motion of the superfluid is affected by the so-called entrainment effect. This brief survey provides an introduction to -- along with a commentary on our current understanding of -- these dynamical aspects, paying particular attention to the role of entrainment, and outlines the impact of superfluidity on neutron-star seismology.