Retrieving the abundances of the heavy elements in spectra of neutron star mergers with SPARK

Nicholas Vieira ( Université McGill )

What is the site of rapid neutron capture (the r-process), which produces most of the elements (atomic number Z >~ 42) in the periodic table? Neutron star - neutron star (NS-NS) and neutron star - black hole (NS-BH) mergers, which produce gravitational waves and shine across the electromagnetic spectrum, are a leading candidate site. Of particular interest in the electromagnetic signal is the UV/optical/IR transient kilonova, which fades in hours/days/weeks, respectively. In kilonovae, freshly-synthesized rapid neutron capture elements imprint absorption features on optical spectra, as observed in AT2017gfo, the UV/optical/IR counterpart to the NS-NS merger GW170817. These spectral features provide insights into the physical conditions of the r-process and the the presence of heavy elements in the merger ejecta, but measuring the detailed composition of the ejecta is challenging. Vieira et al. (2023) introduced Spectroscopic r-Process Abundance Retrieval for Kilonovae (SPARK), a tool for performing inference on kilonova spectra to (1) retrieve elemental abundance patterns, and (2) associate individual absorption features with particular species in the early-time, optically-thick spectra. This first work investigted the 1.4 day post-merger spectrum of AT2017gfo. In Vieira et al. (2024), we extend to 2.4, 3.4, and 4.4 days. At 1.4 and 2.4 days, we see the presence of a bluer kilonova dominated by lighter r-process elements such as strontium, yttrium, and zirconium. This followed by the emergence of a redder component containing heavier elements, such as the lanthanide cerium, at 3.4 days. The presence of this redder component at 3.4 and 4.4 days has important implications for the ability of these mergers to synthesize the heavy elements as we see them in the Solar System and in our home galaxy.