# GW170817$-$the first observed neutron star merger and its kilonova: implications for the astrophysical site of the r-process

## TL;DR

GW170817 was the first observed neutron star merger with electromagnetic counterparts, providing key insights into r-process nucleosynthesis and suggesting collapsars as the dominant source of heavy element enrichment in the galaxy.

## Contribution

This paper links GW170817 observations to r-process element production, proposing collapsars as the main astrophysical site for heavy element synthesis in the galaxy.

## Key findings

- GW170817's kilonova emission is consistent with r-process nucleosynthesis.
- Collapsars may be the dominant source of galactic r-process elements.
- Challenges exist for merger-only models to explain all r-process enrichment.

## Abstract

The first neutron star (NS) merger observed by advanced LIGO and Virgo, GW170817, and its fireworks of electromagnetic counterparts across the entire electromagnetic spectrum marked the beginning of multi-messenger astronomy and astrophysics with gravitational waves. The ultraviolet, optical, and near-infrared emission was consistent with being powered by the radioactive decay of nuclei synthesized in the merger ejecta by the rapid neutron capture process (r-process). Starting from an outline of the inferred properties of this 'kilonova' emission, I discuss possible astrophysical sites for r-process nucleosynthesis in NS mergers, arguing that the heaviest r-process elements synthesized in this event most likely originated in outflows from a post-merger accretion disk. I compare the inferred properties of r-process element production in GW170817 to current observational constraints on galactic heavy r-process nucleosynthesis and discuss challenges merger-only models face in explaining the r-process content of our galaxy. Based on the observational properties of GW170817 and recent theoretical progress on r-process nucleosynthesis in collapsars, I then show how GW170817 points to collapsars as the dominant source of r-process enrichment in the Milky Way. These rare core-collapse events arguably better satisfy existing constraints and overcome problems related to r-process enrichment in various environments that NS mergers face. Finally, I comment on the universality of the r-process and on how variations in light r-process elements can be obtained both in NS mergers and collapsars.