Bulk Viscosity of Relativistic npeμ Matter in Neutron-Star Mergers

We discuss the bulk viscosity of hot and dense npe mu matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with T <= T-t(r) ( T-t(r) similar or equal to 5 divided by 10 MeV is the neutrino-trapping temperature); and (b) the neutrino-trapped regime with T >= T-t(r). 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 similar or equal to 5 divided by 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.

Automated summary

In this study, we investigate the bulk viscosity of hot and dense npe mu matter, focusing on the neutrino-transparent and neutrino-trapped regimes. Using a relativistic density functional approach, we model the nuclear matter with density-dependent parametrization DDME2. Our results show that the bulk viscosity reaches its maximum value at lower temperatures in the neutrino-transparent regime and then decreases rapidly at higher temperatures with neutrino-trapping. As an astrophysical application, we estimate the damping timescales of density oscillations in neutron star mergers and find that the bulk viscosity can significantly affect the post-merger object's evolution at certain temperatures.