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.

Collapsars as Sites of r-process Nucleosynthesis: Systematic Photometric Near-infrared Follow-up of Type Ic-BL Supernovae

“…The association of a GRB with an SN Ic-BL could point toward a central engine that harbors high enough accretion rates to potentially generate r-process elements. Barnes & Duffell (2023) also find that hydrodynamical mixing between the r-process-enriched and r-process-free layers of collapsar increases with wind mass and duration, suggesting that SNe accompanying the longest duration long GRBs may be the most promising sites to search for obvious r-process signatures.…”

The Effects of r-Process Enrichment in Hydrogen-rich Supernovae

“…We take χ = 0.2, in order to give a relatively sharp transition from r-enriched layers at M = M r,mix to r-process-free layers at M ? M r,mix (see Barnes & Duffell 2023 for motivation), though our results are not overly sensitive to this precise value, as long as the transition remains sharp (as shown in Appendix B.3). Rather than M r,mix , it will often be more convenient to quote the ejecta radial mixing fraction,…”

“…For example, the strong wavelength dependence of the bound-bound opacities of the lanthanide group elements could potentially reveal a favorable increase in infrared range excess (e.g., an increase in R − K values) when multigroup radiative transfer is implemented. In addition, multidimensional (magneto-)hydrodynamic simulations of the interaction between the r-process-enriched wind or jet from the central compact object, and the expanding SN ejecta, could better inform our assumptions about the radial and angular distribution of the r-process material within the ejecta (e.g., Barnes & Duffell 2023). Such simulations can be used to constrain the f mix parameter to M r and other properties of the stellar explosion, and eventually inform the (likely nonspherical) distribution of r-process enrichment within the ejecta to motivate multidimensional transport simulations.…”

“…In contrast, most r-process production in an SN explosion will be seeded from the innermost layers and hence is expected to be embedded within the bulk of the ordinary (i.e., non-r-process enriched) ejecta originating from the outer layers of the exploding star. Nevertheless, fast-moving r-process material as generated in the SN explosion, the neutrino-heated wind, or an accretion disk outflow, can in principle be mixed outward through the ejecta via hydrodynamic processes (e.g., Wongwathanarat et al 2015; Barnes & Duffell 2023;, thereby allowing pronounced effects on the SN light curve even at relatively early times as the SN photosphere retreats through renriched outer layers. Barnes & Metzger (2022) developed a semi-analytic onedimensional model for the effects of r-process enrichment on the light curves of stripped-envelope SNe, in which the total mass of r-process elements and their degree of radial mixing through the ejecta are treated as free parameters.…”

A Hubble Space Telescope Search for r-Process Nucleosynthesis in Gamma-Ray Burst Supernovae

“…The SNe Ic-BL associated with long GRBs (GRB-SNe) are generally considered the best targets for observable r-process enrichment in collapsars, in part due to the high angular momentum required to produce large accretion disks capable of launching the GRB jet (e.g., MacFadyen & Woosley 1999;Siegel et al 2019;Barnes & Duffell 2023). In addition, Barnes & Duffell (2023) find that GRB jets are likely to increase mixing between the inner r-process ejecta and the outer layers, thus producing a more prominent red color. As this effect is likely enhanced closer to the jet axis, GRB-SNe, which have relatively pole-on orientations, are strong candidates for observing r-process signatures.…”