Superfluid hyperon bulk viscosity and the r-mode instability of rotating neutron stars

TL;DR

This paper investigates how hyperon superfluidity influences the bulk viscosity and r-mode instability in rotating neutron stars, finding that superfluid effects do not qualitatively change the instability window.

Contribution

It introduces a multifluid formalism to include superfluid bulk viscosity effects in modeling neutron star r-mode instabilities, a novel approach in this context.

Findings

Superfluid effects slightly increase bulk viscosity damping.

The qualitative features of the r-mode instability window remain unchanged.

Superfluidity does not significantly alter the instability conditions.

Abstract

In order to establish whether the unstable r-modes in a rotating neutron star provide a detectable source of gravitational waves, we need to understand the details of the many dissipative processes that tend to counteract the instability. It has been established that the bulk viscosity due to exotic particles, like hyperons, may be particularly important in this respect. However, the effects of hyperon superfluidity have so far not been fully accounted for. While the associated suppression of the reaction rates that give rise to the bulk viscosity has been estimated, superfluid aspects of the fluid dynamics have not been considered. In this paper we determine the r-mode instability window for a neutron star with a $\Sigma^{-}$ hyperon core, using the appropriate multifluid formalism including, for the first time, the effect of the "superfluid" bulk viscosity coefficients. We demonstrate that, even though the extra terms may increase the bulk viscosity damping somewhat, their presence does not affect the qualitative features of the r-mode instability window.