Creation of the heavy elements seen in the light and gravitational waves of a neutron star merger

The remarkable discovery of the gravitational wave event GW170817 and its associated electromagnetic (EM) counterpart provides our first opportunity to dissect the physics of merging neutron stars and address the long standing question of the origin of the elements heavier than iron. Theoretical modeling has suggested that matter ejected in the violent merger of neutron stars may assemble into heavy isotopes in a process of rapid neutron capture ("r-process") nucleosynthesis. The radioactive decay of these isotopes has been theorized to power a distinctive thermal EM glow (a "kilonova"). In this talk, I will review our theoretical understanding of mergers, and compare the expected signals to the extraordinary “multi-messenger” data now obtained for GW170817. Observations at optical through infrared wavelengths closely resemble theoretical predictions of a kilonova, and allow us to infer the production of two spatially distinct components of ejecta, one composed of lighter and one of heavier r-process material. Estimating a merger rate, the inferred mass ejected implies that mergers are a dominant mode of r-process production in the universe. This marks the emergence of a new field of astrophysics, whereby we can constrain the dynamics of dense matter and analyze the signatures of heavy elements at their production site.​