Hunting for the Heaviest Elements with Multi-Messenger Astronomy

The merger of a neutron star with a second neutron star or a black hole produces a quasi-thermal transient powered by the radioactive decay of unstable nuclei assembled during the merger by rapid neutron capture (the r-process). This emission, called a kilonova, is a promising electromagnetic (EM) counterpart to the gravitational wave signals produced by such mergers. Observations of kilonovae allow the determination of the mass and composition of the material ejected by a merger, which is important for constraining the neutron star equation of state and understanding the astrophysical origin of r-process elements. However, interpreting observations requires robust models of kilonova emission, and the construction of such models is challenging due to the complexity of the r-process and the lack of relevant experimental data. I will present theoretical work that enabled the development of detailed and accurate kilonova models. I will discuss the optical properties of r-process material, and show how theoretically-derived synthetic r-process opacities changed our expectation of kilonova colors, provided a tool to differentiate light from heavy regimes of r-process nucleosynthesis, and facilitated the development of multi-component kilonova models. Next, I will show how modeling the transport of radioactive decay products in the kilonova ejecta improved predictions of kilonova light curves and established a connection between bolometric luminosity and the details of r-process radioactive decay. Finally, I will review the role of these models in the interpretation of the EM emission from the first detected neutron star merger, GW170817.