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

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.-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 Myr(-1) 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.