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.
All Science Journal Classification (ASJC) codes
- Nuclear and High Energy Physics