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 ν_e, ν_{¯ e} and ν_{μ ,τ}, respectively) the transition between diffusion and free-streaming conditions occurs around 10¹¹g cm^-3 for all neutrino species. Weak and thermal equilibrium freeze-out occurs deeper (several 10¹²g cm^-3 ) for heavy-flavor neutrinos than for ν_{¯ e} and ν_e (≳10¹¹g cm^-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 (∼3 MeV ) decouple at ρ ∼10¹³g cm^-3 , T ∼10 MeV and Y_e≲0.1 close to weak equilibrium, high-energy ones (∼50 MeV ) decouple from the disk at ρ ∼10⁹g cm^-3 , T ∼2 MeV and Y_e≳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 (≲10¹²g cm^-3 ) in BH-torus systems does not allow softer neutrinos to thermalize and diffuse.