Transport properties of superfluid phonons in neutron stars

TL;DR

This paper reviews the transport properties of superfluid phonons in neutron star cores, focusing on shear and bulk viscosities and thermal conductivity, and compares phonon contributions with other processes affecting neutron star dynamics.

Contribution

It provides a comprehensive analysis of superfluid phonon transport coefficients in neutron stars, including their effects on viscosities, thermal conductivity, and implications for neutron star stability.

Findings

Phonon collisions significantly contribute to bulk viscosities in neutron stars.

Thermal conductivity from phonons dominates over electron-muon interactions below the transition temperature.

Phonon interactions influence the r-mode instability and neutrino emissivity in neutron stars.

Abstract

We review the effective field theory associated with the superfluid phonons that we use for the study of transport properties in the core of superfluid neutrons stars in their low temperature regime. We then discuss the shear and bulk viscosities together with the thermal conductivity coming from the collisions of superfluid phonons in neutron stars. With regard to shear, bulk, and thermal transport coefficients, the phonon collisional processes are obtained in terms of the equation of state and the superfluid gap. We compare the shear coefficient due to the interaction among superfluid phonons with other dominant processes in neutron stars, such as electron collisions. We also analyze the possible consequences for the r-mode instability in neutron stars. As for the bulk viscosities, we determine that phonon collisions contribute decisively to the bulk viscosities inside neutron stars. For the thermal conductivity resulting from phonon collisions, we find that it is temperature independent well below the transition temperature. We also obtain that the thermal conductivity due to superfluid phonons dominates over the one resulting from electron-muon interactions once phonons are in the hydrodynamic regime. As the phonons couple to the $Z$ electroweak gauge boson, we estimate the associated neutrino emissivity. We also briefly comment on how the superfluid phonon interactions are modified in the presence of a gravitational field or in a moving background.