Entrainment in Superfluid Neutron Star Crusts: Hydrodynamic Description and Microscopic Origin

TL;DR

This paper reviews the hydrodynamic modeling of superfluid neutron star crusts, emphasizing the microscopic origin of entrainment and its impact on superfluid dynamics, using a covariant action principle and nuclear energy density functional theory.

Contribution

It introduces a fully covariant hydrodynamical model for neutron star crust superfluidity and clarifies the microscopic origin of entrainment using nuclear energy density functional theory.

Findings

Hydrodynamic model based on a covariant action principle

Microscopic origin of entrainment explained via nuclear energy density functional theory

Comparison of different entrainment estimation methods

Abstract

In spite of the absence of viscous drag, the neutron superfluid permeating the inner crust of a neutron star cannot flow freely, and is entrained by the nuclear lattice similarly to laboratory superfluid atomic gases in optical lattices. The role of entrainment on the neutron superfluid dynamics is reviewed. For this purpose, a minimal hydrodynamical model of superfluidity in neutron-star crusts is presented. This model relies on a fully four-dimensionally covariant action principle. The equivalence of this formulation with the more traditional approach is demonstrated. In addition, the different treatments of entrainment in terms of dynamical effective masses or superfluid density are clarified. The nuclear energy density functional theory employed for the calculations of all the necessary microscopic inputs is also reviewed, focusing on superfluid properties. In particular, the microscopic origin of entrainment and the different methods to estimate its importance are discussed.