Neutrino Fast Flavor Conversions in Neutron-star Post-Merger Accretion Disks

TL;DR

This study uses advanced simulations to show that neutrino fast flavor conversions in neutron-star merger disks significantly influence the neutron richness of outflows, impacting heavy element synthesis and kilonova emissions.

Contribution

It introduces a dynamic neutrino flavor conversion formalism in general-relativistic MHD simulations of post-merger disks, revealing their role in heavy element production.

Findings

Neutrino flavor oscillations are ubiquitous in the disk environment.

Flavor conversions lead to more neutron-rich outflows.

Results suggest disks are key sites for heavy r-process element synthesis.

Abstract

A compact accretion disk may be formed in the merger of two neutron stars or of a neutron star and a stellar-mass black hole. Outflows from such accretion disks have been identified as a major site of rapid neutron-capture (r-process) nucleosynthesis and as the source of 'red' kilonova emission following the first observed neutron-star merger GW170817. We present long-term general-relativistic radiation magnetohydrodynamic simulations of a typical post-merger accretion disk at initial accretion rates of $\dot{M}\sim 1\,M_\odot\,\text{s}^{-1}$ over 400ms post-merger. We include neutrino radiation transport that accounts for effects of neutrino fast flavor conversions dynamically. We find ubiquitous flavor oscillations that result in a significantly more neutron-rich outflow, providing lanthanide and 3rd-peak r-process abundances similar to solar abundances. This provides strong evidence that post-merger accretion disks are a major production site of heavy r-process elements. A similar flavor effect may allow for increased lanthanide production in collapsars. The formalism presented here may also be used in simulations of core-collapse supernovae to explore whether fast conversions strengthen or weaken the explosion.