Thermodynamics conditions of matter in the neutrino decoupling region during neutron star mergers

In this work we investigate the thermodynamics conditions at which neutrinos decouple from matter in neutron star merger remnants by post-processing results of merger simulations. We find that the matter density and the neutrino energies are the most relevant quantities in determining the decoupling surface location. For mean energy neutrinos (∼ 9, 15 and 24 MeV for νe, ν¯ e and νμ,τ, respectively) the transition between diffusion and free-streaming conditions occurs around 1011gcm-3 for all neutrino species. Weak and thermal equilibrium freeze-out occurs deeper (several 1012gcm-3) for heavy-flavor neutrinos than for ν¯ e and νe (≳1011gcm-3). Decoupling temperatures are broadly in agreement with the average neutrino energies, with softer equations of state characterized by ∼ 1 MeV larger decoupling temperatures. Neutrinos streaming at infinity with different energies come from different remnant parts. While low-energy neutrinos (∼3MeV) decouple at ρ∼1013gcm-3, T∼10MeV and Ye≲ 0.1 close to weak equilibrium, high-energy ones (∼50MeV) decouple from the disk at ρ∼109gcm-3, T∼2MeV and Ye≳ 0.25. The presence of a massive NS or a BH influences the neutrino thermalization. While in the former case decoupling surfaces are present for all relevant energies, the lower maximum density (≲1012gcm-3) in BH-torus systems does not allow softer neutrinos to thermalize and diffuse.