Nucleosynthesis in the Ejecta of Neutron Star Mergers

TL;DR

This study investigates nucleosynthesis in neutrino-driven winds from neutron star mergers, revealing angle-dependent yields that complement dynamic ejecta and contribute to heavy element formation.

Contribution

It provides the first detailed analysis of nucleosynthesis in neutrino-driven winds from neutron star mergers using hydrodynamical simulations and reaction network post-processing.

Findings

Electron fraction varies between 0.1 and 0.4 across angles.

Neutrino-driven winds produce lighter heavy elements, complementing dynamic ejecta.

Wind ejecta do not undergo a robust r-process, with yields depending on ejection angle.

Abstract

Heavy elements like gold, platinum or uranium are produced in the r-process, which needs neutron-rich and explosive environments. Neutron star mergers are a promising candidate for an r-process site. They exhibit three different channels for matter ejection fulfilling these conditions: dynamic ejecta due to tidal torques, neutrino-driven winds and evaporating matter from the accretion disk. We present a first study of the integrated nucleosynthesis for a neutrino-driven wind from a neutron star merger with a hyper-massive neutron star. Trajectories from a recent hydrodynamical simulation are divided into four different angle regions and post-processed with a reaction network. We find that the electron fraction varies around $Y_e \approx 0.1 - 0.4$, but its distribution differs for every angle of ejection. Hence, the wind ejecta do not undergo a robust r-process, but rather possess distinct nucleosynthesis yields depending on the angle range. Compared to the dynamic ejecta, a smaller amount of neutron-rich matter gets unbound, but the production of lighter heavy elements with $A \lesssim 130$ in the neutrino-driven wind can complement the strong r-process of the dynamic ejecta.