Exploring Composition Mixing in Kilonova Ejecta with Ray-by-ray Simulations

TL;DR

This study incorporates composition mixing into ray-by-ray simulations of kilonova ejecta, finding minimal impact on heavy-element yields and light curves, thus suggesting mixing effects are limited in such models.

Contribution

We introduce a gradient-based mixing approximation into ray-by-ray BNSM ejecta simulations, showing limited influence on nucleosynthesis and kilonova observables.

Findings

Mixing occurs where electron fraction changes rapidly.

Heavy-element yields are largely unaffected by mixing.

Kilonova light curves show minor reddening due to mixing.

Abstract

Binary neutron star merger (BNSM) ejecta are considered a primary repository of $r$-process nucleosynthesis and a source of the observed heavy-element abundances. We implement composition mixing into ray-by-ray radiation-hydrodynamic simulations of BNSM ejecta, coupled with an online nuclear network (NN). We model mixing via a gradient-based mixing approximation that evolves simultaneously with the hydrodynamics. We find that mixing occurs in regions where the electron fraction changes rapidly. While mixing smooths composition gradients in transition regions, it has a negligible impact on the heavy-element yields. This is because the primary $r$-process site (the equatorial ejecta) is initially homogeneous in free neutrons, leaving no strong gradients for mixing to act upon. In each angular ray, the abundances of the most produced elements are robust under mixing, while the less abundant ones are more affected. The total global abundances change only slightly from mixing, since each angular ray contributes its most abundant elements. Furthermore, the predicted kilonova light curves show only minor reddening, with differences below the detectability of state-of-the-art telescopes. In general, we do not observe significant effects from mixing in the time span of the $r$-process. Consequently, mixing only leads to minor variations in abundances and light curves in ray-by-ray simulations.