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

摘要

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