Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). The transients, named ‘macronovae’ or ‘kilonovae’, are believed to be cradles of production of rare elements such as gold and platinum8. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although suggestive findings of bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 (ref. 12) and γ-ray burst GRB 170817A (refs 13, 14) associated with a galaxy at a distance of 40 megaparsecs. Using a series of spectra from ground-based observatories covering the ultraviolet through the near-infrared wavelength range, we find that the kilonova can be characterized by rapidly expanding ejecta with line features similar to those predicted in current models. The ejecta are optically thick early on, with a velocity of about 0.2 times light speed, reaching a radius of about 50 astronomical units in only 1.5 days. As the ejecta expand, atomic species imprint broad absorption-like lines on the spectral continuum, the products of nucleosynthesis occurring in a post-merger fast-moving (0.2 times light speed) dynamical ejecta and two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03–0.05 solar masses, including high-opacity lanthanides