2025MNRAS.543.2534R

The origin of heavy r-process elements in the Universe is still a matter of great debate, with a confirmed scenario being neutron star (NS) mergers. Additional relevant sites could be specific classes of events, such as gamma-ray burst (GRB) supernova, short-plural form = SNe, long-plural form = supernovae (SNs), where a central engine could push neutron-rich material outwards, contributing to the ejecta of the massive exploding star. Here, we investigate our ability to infer the production of heavy elements in such scenarios, on the basis of the observed nebular emission. We solve the steady-state ionization, level population, and thermal balance, for optically thin ejecta in non-local thermodynamic equilibrium (NLTE), in order to explore the role of heavy elements in cooling the gas, and their imprint in the emergent spectrum a few hundreds days post-explosion. We find that heavy elements would be relevant in the cooling process of the nebula only if they account for at least $\sim 1{{\ \rm per\ cent}}$ of the total ejected mass, at the typical kinetic temperatures of a few thousands K. However, even in the absence of such amount, a few $0.1{{\ \rm per\ cent}}$ of the total ejected mass could be instead sufficient to leave a detectable imprint around $\sim 1\!-\!10~{\rm \mu m}$. This wavelength range, which would be relatively clean from features due to light elements, would be instead robustly populated by lines from heavy elements arising from forbidden transitions in their atomic fine structures. Hence, the new generation of telescopes, represented by the James Webb Space Telescope (JWST), will most likely allow for their detection.