# Bulk viscosity of baryonic matter with trapped neutrinos

## TL;DR

This paper investigates the bulk viscosity of dense baryonic matter with trapped neutrinos, revealing how it varies with temperature and composition, and assessing its impact on neutron star mergers and gravitational wave damping.

## Contribution

It models the bulk viscosity in neutrino-trapped dense matter using a relativistic density functional approach, highlighting its temperature dependence and the effects of lepton fraction.

## Key findings

- Bulk viscosity peaks near the neutrino trapping temperature.
- Bulk viscosity decreases as T^{-2} in the neutrino-trapped regime.
- Bulk viscosity is significantly lower than in the neutrino-transparent regime.

## Abstract

We study bulk viscosity arising from weak current Urca processes in dense baryonic matter at and beyond nuclear saturation density. We consider the temperature regime where neutrinos are trapped and therefore have nonzero chemical potential. We model the nuclear matter in a relativistic density functional approach, taking into account the trapped neutrino component. We find that the resonant maximum of the bulk viscosity would occur at or below the neutrino trapping temperature, so in the neutrino trapped regime the bulk viscosity decreases with temperature as $T^{-2}$, this decrease being interrupted by a drop to zero at a special temperature where the proton fraction becomes density-independent and the material scale-invariant. The bulk viscosity is larger for matter with lower lepton fraction, i.e., larger isospin asymmetry. We find that bulk viscosity in the neutrino-trapped regime is smaller by several orders of magnitude than in the neutrino-transparent regime, which implies that bulk viscosity in neutrino-trapped matter is probably not strong enough to affect significantly the evolution of neutron star mergers. This also implies weak damping of gravitational waves emitted by the oscillations of the postmerger remnant in the high-temperature, neutrino-trapped phase of evolution.