Signatures of r-process Enrichment in Supernovae from Collapsars

Abstract

Despite recent progress, the astrophysical channels responsible for rapid neutron capture (r-process) nucleosynthesis remain an unsettled question. Observations of the kilonova following the gravitational-wave-detected neutron star merger GW170817 established mergers as one site of the r-process, but additional sources may be needed to fully explain r-process enrichment in the universe. One intriguing possibility is that rapidly rotating massive stars undergoing core collapse launch r-process-rich outflows off the accretion disks formed from their infalling matter. In this scenario, r-process winds are one component of the supernova (SN) ejecta produced by “collapsar” explosions. We present the first systematic study of the effects of r-process enrichment on the emission from collapsar-generated SNe. We semianalytically model r-process SN emission from explosion out to late times and determine its distinguishing features. The ease with which r-process SNe can be identified depends on how effectively wind material mixes into the initially r-process-free outer layers of the ejecta. In many cases, enrichment produces a near-infrared (NIR) excess that can be detected within ∼75 days of explosion. We also discuss optimal targets and observing strategies for testing the r-process collapsar theory, and find that frequent monitoring of optical and NIR emission from high-velocity SNe in the first few months after explosion offers a reasonable chance of success while respecting finite observing resources. Such early identification of r-process collapsar candidates also lays the foundation for nebular-phase spectroscopic follow-up in the NIR and mid-infrared, for example, with the James Webb Space Telescope.