Bulk Viscosity of Relativistic npeμ Matter in Neutron-Star Mergers

Abstract

We discuss the bulk viscosity of hot and dense npeμ matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with T≤Ttr (Ttr≃5÷10 MeV is the neutrino-trapping temperature); and (b) the neutrino-trapped regime with T≥Ttr. Nuclear matter is modeled in relativistic density functional approach with density-dependent parametrization DDME2. The maximum of the bulk viscosity is achieved at temperatures T≃5÷6 MeV in the neutrino-transparent regime, then it drops rapidly at higher temperatures where neutrino-trapping occurs. As an astrophysical application, we estimate the damping timescales of density oscillations by the bulk viscosity in neutron star mergers and find that, e.g., at the oscillation frequency f=10 kHz, the damping will be very efficient at temperatures 4≤T≤7 MeV where the bulk viscosity might affect the evolution of the post-merger object.