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

Andrea Endrizzi; Albino Perego; Francesco M. Fabbri; Lorenzo Branca; David Radice; Sebastiano Bernuzzi; Bruno Giacomazzo; Francesco Pederiva; Alessandro Lovato

doi:10.1140/epja/s10050-019-00018-6

2024 Impact factor 2.8

Hadrons and Nuclei

First joint gravitational wave and electromagnetic observations: Implications for nuclear and particle physics

Open Access

Eur. Phys. J. A (2020) 56: 15
https://doi.org/10.1140/epja/s10050-019-00018-6

Regular Article - Theoretical Physics

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

Andrea Endrizzi¹^*, Albino Perego²^,3, Francesco M. Fabbri¹, Lorenzo Branca⁴, David Radice⁵^,6, Sebastiano Bernuzzi¹, Bruno Giacomazzo⁴^,7, Francesco Pederiva²^,7 and Alessandro Lovato⁷^,8

¹ Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität Jena, 07743, Jena, Germany
² Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123, Trento, Italy
³ Istituto Nazionale di Fisica Nucleare, Sezione di Milano-Bicocca, Piazza della Scienza, 20100, Milano, Italy
⁴ Dipartimento di Fisica G. Occhialini, Università di Milano-Bicocca, Piazza della Scienza 3, 20126, Milano, Italy
⁵ Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ, 08540, USA
⁶ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544, USA
⁷ INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Via Sommarive 14, 38123, Trento, Italy
⁸ Physics Division, Argonne National Laboratory, Argonne, IL, 60439, USA

^* e-mail: andrea.endrizzi@uni-jena.de

Received: 18 August 2019
Accepted: 11 November 2019
Published online: 21 January 2020

Abstract

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 ( $\sim$ 9, 15 and 24 MeV for $\nu _e$ , $\bar{\nu }_e$ and $\nu _{\mu ,\tau }$ , respectively) the transition between diffusion and free-streaming conditions occurs around $10^{11}\mathrm{g}~\mathrm{cm}^{-3}$ for all neutrino species. Weak and thermal equilibrium freeze-out occurs deeper (several $10^{12}\mathrm{g}~\mathrm{cm}^{-3}$ ) for heavy-flavor neutrinos than for $\bar{\nu }_e$ and $\nu _e$ ( $\gtrsim 10^{11}\mathrm{g}~\mathrm{cm}^{-3}$ ). Decoupling temperatures are broadly in agreement with the average neutrino energies, with softer equations of state characterized by $\sim$ 1 MeV larger decoupling temperatures. Neutrinos streaming at infinity with different energies come from different remnant parts. While low-energy neutrinos ( $\sim 3~\mathrm{MeV}$ ) decouple at $\rho \sim 10^{13}\mathrm{g}~\mathrm{cm}^{-3}$ , $T \sim 10~\mathrm{MeV}$ and $Y_e \lesssim 0.1$ close to weak equilibrium, high-energy ones ( $\sim 50~\mathrm{MeV}$ ) decouple from the disk at $\rho \sim 10^{9}\mathrm{g}~\mathrm{cm}^{-3}$ , $T \sim 2~\mathrm{MeV}$ and $Y_e \gtrsim 0.25$ . 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 ( $\lesssim 10^{12}\mathrm{g}~\mathrm{cm}^{-3}$ ) in BH-torus systems does not allow softer neutrinos to thermalize and diffuse.