GW170817: Optical and Infrared Observations and Kilonova

The first gravitational wave (GW) observation from a neutron star merger was successfully made for GW170817. The detection triggered electromagnetic (EM) wave observations over the entire wavelength range, which enabled the first identification of an EM counterpart of a GW source. In optical and infrared wavelengths, the counterpart shows characteristic properties of "kilonova", EM emission powered by radioactive decays of newly synthesized r-process elements. In this talk, I present optical and infrared observations of GW170817 and summarize what we have learned about r-process nucleosynthesis by neutron star mergers. Then, I highlight open questions and future prospects toward understanding the origin of r-process elements in the Universe.