Cristina Manuel, Jaume Tarrus, Laura Tolos
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 see that the static and frequency-dependent bulk viscosity coefficients due to phonons are distinguishable for $n \gtrsim 4n_0$ depending on the model used for the neutron gap. 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.
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http://arxiv.org/abs/1302.5447
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