Enrichment history of r-process elements shaped by a merger of neutron star pairs

TL;DR

This study supports neutron star mergers as the primary source of heavy r-process elements in the galaxy, aligning with observed abundance patterns and predicting detectable gravitational waves from such events.

Contribution

It demonstrates that neutron star mergers at a Galactic rate of 12-23 per million years can explain r-process element distribution in various galactic environments.

Findings

Consistent r-process abundance ratios in dwarf galaxies and a star in Sculptor.

Large scatter in r-process to iron ratios explained by assembly of protogalactic fragments.

Predicted high detection rate of gravitational waves from neutron star mergers.

Abstract

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration gamma-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A>110) by recent numerical simulations. However, the properties characterizing NSM events - their rarity and high yield of r-process elements per event - have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12-23 per Myr are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.