Identification of Sr in the spectrum of the kilonova AT2017gfo

The rapid (r) neutron capture process that creates half of all the elements heavier than iron was first postulated six decades ago. But the event or events in which the r-process creates the heavy elements is still not spectroscopically confirmed. Neutron star mergers are prime candidates for the main site of the r-process. The kilonova, AT2017gfo, emerging from the gravitational-wave–discovered neutron star merger, GW170817, was the first kilonova where detailed spectra were recorded. AT2017gfo was argued to have properties broadly consonant with this hypothesis, however, no robust spectroscopic identification of neutron-capture elements has thus far been made. We identify the neutron-capture element strontium in the spectra of AT2017gfo. This identification of a neutron-capture element associated with the collision of two extreme-density stars firmly establishes the origin of r-process elements