Isospin Equilibration in Neutron Star Mergers

We analyze the isospin equilibration properties of neutrino-transparent nuclear ($npe$) matter in the temperature and density range that is relevant to neutron star mergers. Our analysis incorporates neutrino-transparency corrections to the isospin (``beta'') equilibrium condition which become noticeable at $T\gtrsim 1\,$MeV. We find that the isospin relaxation rate rises rapidly as temperature rises, and at $T\approx 5\,$MeV it is comparable to the timescale of the density oscillations that occur immediately after the merger. This produces a resonant peak in the bulk viscosity at $T\sim 5\,$MeV, which causes density oscillations to be damped on the timescale of the merger. We conclude that there is good reason to include isospin relaxation dynamics in merger simulations.