Cake Talk by Nick Vieira
What is the origin of the heaviest elements in the Universe? It is thought that most elements with atomic number Z > 30 are produced in large part by rapid neutron capture: r-process nucleosynthesis. Mergers of neutron stars (NS-NS) or a neutron star and black hole (NS-BH) are a prime candidate for the site of the r-process. After producing gravitational waves, these mergers shine across the electromagnetic spectrum. Of particular is the UV/optical/IR transient kilonova, which fades in hours/days/weeks, respectively. These extraordinarily fast transients are powered by the decay of radioactive elements produced by the r-process. The optical/IR spectra of these kilonovae should show signs of the presence of these heavy elements. One such kilonovae, that which accompanied the NS-NS merger GW170817, is among the most intensely studied events in astrophysics. The kilonova following the merger showed clear evidence for the synthesis of radioactive r-process elements. But precisely how much of each element was synthesized? Here, I will discuss our tool Spectroscopic r-Process Abundance Retrieval for Kilonovae (SPARK) which fits the spectra of kilonovae to directly infer the abundances of the elements produced in the kilonova ejecta. We have applied SPARK to the optical/IR spectra of GW170817 at 1.4, 2.4, 3.4, and 4.4 days post-merger. We see the presence of a bluer kilonova dominated by lighter r-process elements such as strontium at early times, followed by the emergence of a redder component containing heavier elements, such as the lanthanide cerium, as of 3.4 days. The emergence of this new redder component at 3.4 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. Finally, I will discuss current work on the later epochs of GW170817, the potential importance of non-(local thermodynamic equilibrium) effects at these later epochs, and applications to future kilonovae.