Detecting the Synthesis of the Heaviest Elements with Photometry of a Kilonova in the Optically Thin Phase

Approximately half of all elements heavier than iron form through rapid-neutron capture. Yet the cosmic origin of these "r-process" elements has been debated for over 60 years. In 2017, the discovery of a kilonova associated with the gravitational wave source GW170817 partially unraveled this mystery---firmly establishing that neutron star mergers do synthesize r-process elements. However, in this discovery's wake many questions remain. In particular, it is unclear whether GW170817 synthesized any of the heaviest "third peak" or actinide elements. As a result, we are still uncertain whether NS mergers are the only – or even the dominant – site of r-process production. We propose to use JWST/NIRCam and JWST/MIRI to tackle this open question by observing a new kilonova discovered during LIGO/Virgo/KAGRA Observing Run 4. We will carry out our observations between ~30-120 days post-merger when the ejecta is optically thin. During this nebular phase, the bolometric luminosity will trace the instantaneous heating rate due to radioactive decay; with decline rates that vary depending how far up the periodic table the r-process proceeded. Constraining the bolometric luminosity at these epochs requires broad-band coverage between ~1-10 microns to depths of ~25-28 mag (AB). Hence, JWST is the only facility capable of carrying out these observations.