Neutron Star Mergers as sites of r-process Nucleosynthesis and Short Gamma-Ray Bursts

TL;DR

This paper reviews evidence supporting neutron star mergers as key sites for heavy r-process element creation and their connection to short gamma-ray bursts, highlighting recent gravitational wave observations and chemical evolution modeling challenges.

Contribution

It provides a comprehensive overview of observational evidence linking neutron star mergers to r-process nucleosynthesis and discusses the complexities in modeling their role in Galactic chemical evolution.

Findings

GW170817 confirmed neutron star mergers produce r-process elements.

Current models struggle to match europium abundance in stars with [Fe/H] > -1.

Multiple stellar populations are needed to explain chemical evolution observations.

Abstract

Neutron star mergers have been long considered as promising sites of heavy $r$-process nucleosynthesis. We overview observational evidence supporting this scenario including: the total amount of $r$-process elements in the Galaxy, extreme metal poor stars, geological radioactive elemental abundances, dwarf galaxies, and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical, and near infrared observations point to $r$-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We find however that, within simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the europium abundance history of Galactic stars for [Fe/H] $\gtrsim -1$. This implies that to account for the role of mergers in the Galactic chemical evolution, we need a Galactic model with multiple populations that have different spatial distributions and/or varying formation rates.