Eur. Phys. J. A (2019) 55: 124
https://doi.org/10.1140/epja/i2019-12810-7

Regular Article - Theoretical Physics

Thermodynamics conditions of matter in neutron star mergers

¹ Department of Physics, Trento University, Via Sommarive 14, 38123, Trento, Italy
² Istituto Nazionale di Fisica Nucleare, Sezione di Milano-Bicocca, Piazza della Scienza, 20100, Milano, Italy
³ Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität Jena, 07743, Jena, Germany
⁴ Institute for Advanced Study, 1 Einstein Drive, 08540, Princeton, NJ, USA
⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, 08544, Princeton, NJ, USA

^* e-mail: albino.perego@unitn.it

Received: 19 March 2019
Accepted: 18 June 2019
Published online: 2 August 2019

Abstract

Matter in neutron star collisions reaches densities up to few times the nuclear saturation threshold, and temperatures up to one hundred MeV. Understanding the structure and composition of such matter requires many-body non-perturbative calculations that are currently highly uncertain. Unique constraints on the neutron star matter are provided by gravitational-wave observations aided by numerical relativity simulations. In this work, we explore the thermodynamical conditions of matter along the merger dynamics. We consider 3 microphysical equations of state and numerical relativity simulations including approximate neutrino transport. The neutron star cores collision and their multiple bounces heat the initially cold matter to several tens of MeV. Streams of hot matter with initial densities move outwards and cool due to decompression and neutrino emission. The merger can result in a neutron star remnant with densities up to and temperatures MeV. The highest temperatures are confined in an approximately spherical annulus at densities . Such temperatures favour positron-neutron capture thus leading to a neutrino emission dominated by electron antineutrinos. We study the impact of trapped neutrinos on the remnant matter’s pressure, electron fraction and temperature and find that it has a negligible effect. Disks around neutron star or black hole remnant are neutron rich and not isentropic, but they differ in size, entropy and lepton fraction depending on the nature of the central object. In the presence of a black hole, disks are smaller and mostly transparent to neutrinos; in presence of a massive neutron star, they are more massive, geometrically and optically thick.