Shear viscosity in neutron star cores

TL;DR

This paper calculates the shear viscosity in neutron star cores considering various particle interactions, Landau damping effects, and superfluidity, providing a comprehensive model for different equations of state.

Contribution

It introduces a detailed calculation of shear viscosity in neutron star cores, including Landau damping and in-medium effects, with formulas applicable to any nucleon dense matter equation of state.

Findings

Landau damping lowers electron-muon viscosity and causes a T^{-5/3} temperature dependence.

Both electron-muon and neutron viscosities are significant and depend on superfluidity.

The results are applicable across various dense matter equations of state.

Abstract

We calculate the shear viscosity $\eta = \eta_{e\mu}+\eta_{n}$ in a neutron star core composed of nucleons, electrons and muons ($\eta_{e\mu}$ being the electron-muon viscosity, mediated by collisions of electrons and muons with charged particles, and $\eta_{n}$ the neutron viscosity, mediated by neutron-neutron and neutron-proton collisions). Deriving $\eta_{e\mu}$, we take into account the Landau damping in collisions of electrons and muons with charged particles via the exchange of transverse plasmons. It lowers $\eta_{e\mu}$ and leads to the non-standard temperature behavior $\eta_{e\mu}\propto T^{-5/3}$. The viscosity $\eta_{n}$ is calculated taking into account that in-medium effects modify nucleon effective masses in dense matter. Both viscosities, $\eta_{e\mu}$ and $\eta_{n}$, can be important, and both are calculated including the effects of proton superfluidity. They are presented in the form valid for any equation of state of nucleon dense matter. We analyze the density and temperature dependence of $\eta$ for different equations of state in neutron star cores, and compare $\eta$ with the bulk viscosity in the core and with the shear viscosity in the crust.