Probing Heavy-element Inception in Novel Elusive Cosmic Sources

The origin of the heaviest elements beyond iron in the Universe remains an open problem in astrophysics. While binary neutron star mergers are confirmed to produce heavy elements via r-process nucleosynthesis, their long delay times and uncertain yields hint towards the existence of an alternative production. A promising candidate is the collapsar. Here, the collapse of a rapidly rotating massive star forms an accretion disk around a compact remnant, where the r-process can occur. Collapsars produce UVOIR transients: Type Ic-BL supernovae from progenitors of masses 10Msun or higher and super-kilonovae from progenitors above 130Msun. These transients carry r-process signatures in their light curves and spectra, particularly discernible in the late-time infrared signal. With current and upcoming facilities such as JWST, Rubin, and Roman, it is now possible to test their role as r-process sites. However, robust theoretical models of nebular spectra from collapsar transients are needed for that purpose, which is still lacking. The light curves and spectra of these events are governed by the atomic properties of the heavy elements, yet key atomic data are missing. As an MSCA fellow, I will develop the first comprehensive framework for probing heavy-element production in collapsars through nebular spectra. I will generate atomic data with HULLAC and use them in non-local thermal equilibrium radiative transfer simulations with SUMO, computing spectra of Type Ic-BL supernovae and super-kilonovae across a range of progenitor properties, ejecta masses, and compositions. I will use these models to understand observational data and design search and follow-up strategies with Rubin, JWST, and Roman, in collaboration with Prof. Stephen Smartt’s group at Oxford. Hence, this project will significantly advance our understanding of collapsars as a site of heavy elements, leading us towards understanding the origin of the heaviest elements.