# Bulk viscosity of superfluid neutron stars

## TL;DR

This paper calculates four bulk viscosity coefficients in superfluid neutron stars caused by non-equilibrium beta-processes, and assesses their impact on sound wave damping, highlighting their comparable significance.

## Contribution

First calculation of all four bulk viscosity coefficients in superfluid neutron stars due to beta-processes, advancing understanding of their hydrodynamics.

## Key findings

- All four coefficients contribute similarly to sound wave damping.
- Bulk viscosity significantly affects neutron star oscillation damping.
- Results provide estimates for damping times of sound waves.

## Abstract

The hydrodynamics, describing dynamical effects in superfluid neutron stars, essentially differs from the standard one-fluid hydrodynamics. In particular, we have four bulk viscosity coefficients in the theory instead of one. In this paper we calculate these coefficients, for the first time, assuming they are due to non-equilibrium beta-processes (such as modified or direct Urca process). The results of our analysis are used to estimate characteristic damping times of sound waves in superfluid neutron stars. It is demonstrated that all four bulk viscosity coefficients lead to comparable dissipation of sound waves and should be considered on the same footing.