Instability of Superfluid Flow in the Neutron Star Inner Crust

TL;DR

This paper investigates the hydrodynamic stability of superfluid vortices in neutron star crusts, revealing potential instabilities that could lead to turbulence and enhanced dissipation affecting neutron star dynamics.

Contribution

It introduces a stability analysis of vortex creep in neutron star crusts, identifying conditions for instability and potential turbulence in superfluid flow.

Findings

Vortex creep introduces unstable low-frequency modes.

Superfluid flow is unstable over length scales up to 10 meters.

Large dissipation can suppress the instability.

Abstract

Pinning of superfluid vortices to the nuclear lattice of the inner crust of a neutron star supports a velocity difference between the superfluid and the solid as the star spins down. Under the Magnus force that arises on the vortex lattice, vortices undergo {\em vortex creep} through thermal activation or quantum tunneling. We examine the hydrodynamic stability of this situation. Vortex creep introduces two low-frequency modes, one of which is unstable above a critical wavenumber for any non-zero flow velocity of the superfluid with respect to the solid. For typical pinning parameters of the inner crust, the superfluid flow is unstable over length scales $\lap 10$ m and over timescales as fast as months. The vortex lattice could degenerate into a tangle, and the superfluid flow could become turbulent. Unexpectedly large dissipation would suppress this instability.