$r$-process nucleosynthesis from compact binary mergers

TL;DR

This paper discusses how compact binary mergers, such as neutron star collisions, are key sites for the rapid neutron capture process ($r$-process) that creates many of the universe's heavy elements, supported by recent observational evidence.

Contribution

It highlights the role of neutron star mergers in $r$-process nucleosynthesis and connects theoretical predictions with recent observational data like kilonovae.

Findings

Neutron star mergers eject neutron-rich matter capable of forming heavy elements.

The $r$-process occurs efficiently in these ejecta, producing a significant fraction of universe's heavy elements.

Observations of kilonovae confirm mergers as major sites of heavy element formation.

Abstract

The merger of two neutron stars or of a neutron star and a black hole often result in the ejection of a few percents of a solar mass of matter expanding at high speed in space. Being matter coming from the violent disruption of a neutron star, these ejecta are initially very dense, hot and extremely rich in neutrons. The few available protons form heavy nuclei ("seeds") that absorb the more abundant free neutrons, increasing their size. The neutron density is so high that a substantial number of neutron captures occur before the resulting unstable nuclei can decay toward more stable configurations, converting neutrons into protons. Depending mostly on the initial neutron richness, this mechanism leads to the formation of up to half of the heavy elements that we observe in nature and it is called rapid neutron capture process ("$r$-process"). The prediction of the precise composition of the ejecta requires a detailed knowledge of the properties of very exotic nuclei, that have never been produced in a laboratory. Despite having long been a speculative scenario, nowadays several observational evidences point to compact binary mergers as one of the major sites where heavy elements are formed in the Universe. The most striking one was the detection of a kilonova following the merger of a neutron star binary: the light emitted by this astronomical transient is indeed powered by the radioactive decay of freshly synthesized neutron-rich nuclei and testifies the actual nature of compact binary mergers as cosmic forges.