Accurate muonic interactions in neutron star mergers and impact on heavy-element nucleosynthesis

TL;DR

This paper introduces advanced neutron star merger simulations including muonic neutrinos, revealing their significant impact on neutrino emission, ejecta composition, and heavy-element nucleosynthesis, which was previously underestimated.

Contribution

It presents the first BNS merger simulations with five neutrino species, incorporating muonic processes, and demonstrates their influence on postmerger dynamics and element formation.

Findings

Muonic neutrino equilibrium forms, altering neutrino hierarchy.

Muonic processes increase neutrino luminosity and cooling rates.

Ejected mass is smaller and more neutron-rich, affecting nucleosynthesis.

Abstract

The abundances resulting from $r$-process nucleosynthesis as predicted by simulations of binary neutron star (BNS) mergers remain an open question as the current state of the art is still restricted to three-species neutrino transport. We present the first BNS merger simulations employing a moment-based general-relativistic neutrino transport with five neutrino species, thus including (anti)muons and advanced muonic $\beta$-processes, and contrast them with traditional three-neutrino-species simulations. Our results show that a muonic trapped-neutrino equilibrium is established, forming a different trapped-neutrino hierarchy akin to the electronic equilibrium. The formation of (anti)muons and the muonization via muonic $\beta$-processes enhance the neutrino luminosity, leading to rapid cooling in the early postmerger phase. Since muonic processes redirect part of the energy otherwise used for protonization by electronic processes, they yield a cooler remnant and disk, together with neutrino-driven winds that are more neutron-rich. Importantly, the unbound ejected mass is smaller than three-species simulations, and, because of its comparatively smaller temperature and proton fraction, it can enhance lanthanide production and reduce the overproduction of light $r$-process elements for softer equations of state. This finding underlines the importance of muonic interactions and five neutrino species in long-lived BNS remnants.