Hydrodynamic Mixing of Accretion Disk Outflows in Collapsars: Implications for r-process Signatures

Abstract

The astrophysical environments capable of triggering heavy-element synthesis via rapid neutron capture (the r-process) remain uncertain. While binary neutron star mergers (NSMs) are known to forge r-process elements, certain rare supernovae (SNe) have been theorized to supplement—or even dominate— r-production by NSMs. However, the most direct evidence for such SNe, unusual reddening of the emission caused by the high opacities of r-process elements, has not been observed. Recent work identified the distribution of r-process material within the SN ejecta as a key predictor of the ease with which signals associated with r-process enrichment could be discerned. Though this distribution results from hydrodynamic processes at play during the SN explosion, thus far it has been treated only in a parameterized way. We use hydrodynamic simulations to model how disk winds—the alleged locus of r-production in rare SNe—mix with initially r-process-free ejecta. We study mixing as a function of the wind mass, wind duration, and the initial SN explosion energy, and find that it increases with the first two of these and decreases with the third. This suggests that SNe accompanying the longest long-duration gamma-ray bursts are promising places to search for signs of r-process enrichment. We use semianalytic radiation transport to connect hydrodynamics to electromagnetic observables, allowing us to assess the mixing level at which the presence of r-process material can be diagnosed from SN light curves. Analytic arguments constructed atop this foundation imply that a wind-driven r-process-enriched SN model is unlikely to explain standard energetic SNe.