On the Role of Muons in Binary Neutron Star Mergers: First Simulations

TL;DR

This paper presents the first simulations of binary neutron star mergers that include muons in the equation of state and hydrodynamics, revealing their significant impact on the remnant's evolution and ejecta properties.

Contribution

It introduces the inclusion of muons and muonic weak reactions into BNS merger simulations, demonstrating their effects on collapse delay and ejecta characteristics.

Findings

Muons delay gravitational collapse of the remnant.

Muons prevent collapse within simulation time, confining in dense regions.

Ejecta are smaller, neutron-rich, and slower with muons.

Abstract

In this work we present a set of binary neutron star (BNS) merger simulations including the net muon fraction as an additional degree-of-freedom in the equation of state (EoS) and hydrodynamics evolution using the numerical-relativity code BAM. Neutrino cooling is modeled via a neutrinos leakage scheme, including in-medium corrections to the opacities and emission rates of semi-leptonic charged-current reactions, although within the elastic approximation. We show that, for our particular choice of baseline baryonic EoS, the presence of muons delays the gravitational collapse of the remnant compared to the case where muons are neglected. Furthermore, when muons and muonic weak reactions are considered, no gravitational collapse occurs within our simulation time and muons are confined in the densest portions of the remnant, while the disk is effectively colder, less protonized and de-muonized. Accordingly, ejecta properties are affected, e.g., ejecta masses are systematically smaller for the muonic setups and exhibit a larger fraction of neutron-rich, small velocity material. Overall, our results suggest that the inclusion of muons and muon-flavored neutrino reactions in the context of BNS merger simulations should not be neglected, thus representing an important step towards more realistic modeling of such systems.