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

• Published in: The European physical journal. A, Hadrons and nuclei Hadrons and nuclei, 2020, Vol.56 (1), Article 15
• Main Authors: Endrizzi, Andrea, Perego, Albino, Fabbri, Francesco M., Branca, Lorenzo, Radice, David, Bernuzzi, Sebastiano, Giacomazzo, Bruno, Pederiva, Francesco, Lovato, Alessandro
• Format: Article
• Language: English
• Subjects: ASTRONOMY AND ASTROPHYSICS; Decoupling; Density; Equations of state; First joint gravitational wave and electromagnetic observations: Implications for nuclear and particle physics; Hadrons; Heavy Ions; Neutrinos; Neutron stars; Neutrons; Nuclear Fusion; Nuclear Physics; Particle and Nuclear Physics; Physics; Physics and Astronomy; Post-production processing; Regular Article - Theoretical Physics; Star mergers; Stellar evolution; Thermalization (energy absorption); Thermodynamics; Toruses
• ISSN: 1434-6001; 1434-601X
• DOI: 10.1140/epja/s10050-019-00018-6

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 10 11 g cm - 3 for all neutrino species. Weak and thermal equilibrium freeze-out occurs deeper (several 10 12 g cm - 3 ) for heavy-flavor neutrinos than for ν ¯ e and ν e ( ≳ 10 11 g 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 ∼ 10 13 g 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 ∼ 10 9 g 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 ( ≲ 10 12 g cm - 3 ) in BH-torus systems does not allow softer neutrinos to thermalize and diffuse.