Bulk Viscosity of Relativistic npeμ Matter in Neutron-Star Mergers

We discuss the bulk viscosity of hot and dense (Formula presented.) matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with (Formula presented.) ((Formula presented.) MeV is the neutrino-trapping temperature); and (b) the neutrino-trapped regime with (Formula presented.). Nuclear matter is modeled in relativistic density functional approach with density-dependent parametrization DDME2. The maximum of the bulk viscosity is achieved at temperatures (Formula presented.) 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 (Formula presented.) kHz, the damping will be very efficient at temperatures (Formula presented.) MeV where the bulk viscosity might affect the evolution of the post-merger object.