Bulk viscosity of superfluid hyperon stars

TL;DR

This paper extends relativistic hydrodynamics to calculate bulk viscosity in superfluid hyperon matter of neutron stars, revealing sixteen coefficients with only three independent, and corrects a key formula for non-superfluid cases.

Contribution

It introduces a comprehensive framework for calculating bulk viscosity in superfluid hyperon matter, identifying the number of independent coefficients and correcting previous inaccuracies.

Findings

Identified sixteen bulk viscosity coefficients in superfluid hyperon matter.

Only three of these coefficients are independent.

Corrected a widely used formula for non-superfluid nucleon-hyperon matter.

Abstract

We calculated bulk viscosity due to non-equilibrium weak processes in superfluid nucleon-hyperon matter of neutron stars. For that, the dissipative relativistic hydrodynamics, formulated in paper [1] for superfluid mixtures, was extended to the case when both nucleons and hyperons are superfluid. It was demonstrated that in the most general case (when neutrons, protons, Lambda, and Sigma^{-} hyperons are superfluid), non-equilibrium weak processes generate sixteen bulk viscosity coefficients, with only three of them being independent. In addition, we corrected an inaccuracy in a widely used formula for the bulk viscosity of non-superfluid nucleon-hyperon matter.