Neutron star mergers as sites of r -process nucleosynthesis and short gamma-ray bursts

Neutron star mergers have been long considered as promising sites of heavy r-process nucleosynthesis. We overview the 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 (n/R) 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 however, find that, within the 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 Eu abundance history of galactic stars for [Fe/H] ≥-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.