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. consider two regimes interest: (a) neutrino-transparent regime with $T\leq T_{\rm tr}$ ($T_{\rm tr}\simeq 5\div 10$ MeV is neutrino-trapping temperature); (b) neutrino-trapped $T\geq tr}$. Nuclear modeled in relativistic density functional approach density-dependent parametrization DDME2. The maximum achieved at temperatures $T \simeq 6$ regime, then it drops rapidly higher where occurs. As an astrophysical application, we estimate damping timescales oscillations by neutron star mergers find that, e.g., oscillation frequency $f=10$ kHz, will be very efficient $4\leq T\leq 7$ might affect evolution post-merger object.