Vorticity affects the stability of neutron stars

TL;DR

This paper investigates how vorticity influences the stability of neutron stars by modifying viscous damping, potentially allowing colder stars to spin faster without becoming unstable.

Contribution

It introduces a vorticity-dependent correction to shear viscosity in neutron star models, altering the stability diagram at low temperatures.

Findings

Vorticity couples to shear viscosity, affecting damping timescales.

Colder neutron stars can sustain higher spin rates due to vorticity effects.

Abstract

The spin rate \Omega of neutron stars at a given temperature T is constrained by the interplay between gravitational-radiation instabilities and viscous damping. Navier-Stokes theory has been used to calculate the viscous damping timescales and produce a stability curve for r-modes in the (\Omega,T) plane. In Navier-Stokes theory, viscosity is independent of vorticity, but kinetic theory predicts a coupling of vorticity to the shear viscosity. We calculate this coupling and show that it can in principle significantly modify the stability diagram at lower temperatures. As a result, colder stars can remain stable at higher spin rates.