Inferring the abundances of the heavy elements from neutron star mergers with SPARK

Nicholas Vieira ( Université McGill )

What is the cosmic origin of the elements heavier than iron in the periodic table? Most of these elements require exceptionally neutron-rich environments for their synthesis by rapid neutron capture, the r-process. One leading candidate site is the ejecta produced during mergers of neutron stars with each other or with black holes. To date, we have seen one unambiguous neutron star merger: the event GW170817, first detected in gravitational waves, and then across the electromagnetic spectrum. The optical/infrared counterpart to this event, known as a kilonova, yielded a spectrum which closely matched predictions for an event powered by the radioactive decay of these r-process elements. But what is the precise element-by-element abundance pattern of this ejecta---how much of each element was produced? In this talk, I'll present a novel technique for inferring the complete abundance pattern of kilonova ejecta by performing spectral retrieval on optical/infrared spectra. In applying this technique to the optically-thick spectrum of the GW170817 kilonova at 1.4 days post-merger, we extract the first complete abundance pattern for this event. We also associate features in the spectrum with specific elements strontium, yttrium, and zirconium. This tool, named Spectroscopic r-Process Abundance Retrieval for Kilonovae (SPARK) will enable computationally tractable inference of the abundances of future kilonovae detected in the coming years.