Bulk viscosity coefficients due to phonons in superfluid neutron stars

TL;DR

This paper calculates bulk viscosity coefficients due to phonon collisions in superfluid neutron stars using effective field theory, revealing their temperature dependence and dominance over Urca processes at certain densities and temperatures.

Contribution

It introduces a general method to compute phonon-induced bulk viscosities in superfluid systems based on their symmetries and equation of state.

Findings

Bulk viscosity coefficients are dominated by collinear $2\leftrightarrow3$ phonon processes.

At densities above 4$n_0$, coefficients are within 10% of static values for $T\lesssim 10^9$K.

Phonon collisions dominate bulk viscosity at $T\sim 10^9$K except near $2n_0$ where Urca processes are significant.

Abstract

We calculate the three bulk viscosity coefficients as arising from the collisions among phonons in superfluid neutron stars. We use effective field theory techniques to extract the allowed phonon collisional processes, written as a function of the equation of state of the system. The solution of the dynamical evolution of the phonon number density allows us to calculate the bulk viscosity coefficients as function of the phonon collisional rate and the phonon dispersion law, which depends on the neutron pairing gap. Our method of computation is rather general, and could be used for different superfluid systems, provided they share the same underlying symmetries. We find that the behavior with temperature of the bulk viscosity coefficients is dominated by the contributions coming from the collinear regime of the $2\leftrightarrow3$ phonon processes. For typical star radial pulsation frequencies of $\omega \sim 10^{4} s^{-1}$, we obtain that the bulk viscosity coefficients at densities $n \gtrsim 4n_0$ are within 10% from its static value for $T \lesssim 10^9 K$ and for the case of strong neutron superfluidity in the core with a maximum value of the $^3P_2$ gap above 1 MeV, while, otherwise, the static solution is not a valid approximation to the bulk viscosity coefficients. Compared to previous results from Urca and modified Urca reactions, we conclude that at $T\sim 10^9$K phonon collisions give the leading contribution to the bulk viscosities in the core of the neutron stars, except for $n \sim 2n_0$ when the opening of the Urca processes takes place.