Waves and instabilities in dissipative rotating superfluid neutron stars

TL;DR

This paper analyzes wave behavior and instabilities in the superfluid core of rotating neutron stars, emphasizing the effects of mutual friction, vortex tension, and background flows on wave propagation and stability.

Contribution

It provides a detailed theoretical analysis of wave dynamics, including damping and instabilities, in superfluid neutron star cores considering mutual friction and vortex effects.

Findings

Sound waves along vortex arrays are undamped by mutual friction.

Inertial waves are damped by mutual friction regardless of propagation direction.

Background flows can induce dynamical instabilities in inertial waves.

Abstract

We discuss wave propagation in rotating superfluid neutron star cores, taking into account the vortex mediated mutual friction force. For models where the two fluids co-rotate in the unperturbed state, our analysis clarifies the role of chemical coupling and entrainment for sound and inertial waves. We also investigate the mutual friction damping, providing results that demonstrate the well-known fact that sound waves propagating along a vortex array are undamped. We show that the same is not true for inertial waves, which are damped by the mutual friction regardless of the propagation direction. We then include the vortex tension, which arises due to local vortex curvature. Focussing on purely transverse inertial waves, we derive the small correction that the tension induces in the wave frequency. Finally, we allow for a relative linear flow in the background (along the rotation axis). In this case we show how the mutual friction coupling may induce a dynamical instability in the inertial waves. We discuss the critical flow required for the instability to be present, its physical interpretation and the possible relevance it may have for neutron star physics.