Neutron Star Mergers as Sites of Heavy Element Synthesis

Almost three years ago, the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 11 hours, a bright but rapidly fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 130 Million light years. The properties of the optical transient match remarkably well predictions for ``kilonova'' emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by rapid neutron capture nucleosynthesis (r-process). The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei, while other features of the light curve and possible spectral features suggest the joint synthesis of lighter r-process elements. I will describe our understanding of the sources of neutron-rich ejecta in neutron star mergers and the sensitivity of their properties (and the resulting kilonova signal) to the lifetime of the neutron star remnant. I will describe how multi-messenger observations of GW170817 and future mergers constrain the astrophysical origin of the r-process and the properties of~neutron stars (particularly their uncertain radii and maximum mass, which are determined by the equation of state of nuclear matter). Time permitting, I will overview new results from LIGO's ongoing O3 run and preview the upcoming era of multi-messenger astronomy, once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected every few weeks.