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

Abstract

One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producing r-process elements. Simulations have shown that 0.01–0.1 M ⊙ of r-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of r-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity of r-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account for r-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on the r-process mass for these SNe. We also perform independent light curve fits to models without the r-process. We find that the r-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence of r-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities of r-process ejecta mass or indicate whether all collapsars are completely devoid of r-process nucleosynthesis.

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

“…found for a number of SNe Ic-BL found without GRBs (Anand et al 2024). Overall, we find that this GRB-SN provides a poor fit to the models, though the colors are more in line with the trends of the r-process-free case.…”

“…The burst had a gamma-ray duration of 53 s. The early afterglow was detected across the electromagnetic spectrum (e.g., Rhodes et al 2020;H.E.S.S. Collaboration et al 2021;Dichiara et al 2022), and later spectroscopic observations revealed features consistent with an SN Ic-BL (Hu et al 2021;Anand et al 2024).…”

“…The existing suite of models assumes a spherical distribution of ejecta parameterized by total SN ejecta mass (M tot ), 56 Ni ejecta mass (M 56Ni ), average ejecta expansion velocity as a fraction of the speed of light (β ej ), mass of r-process material (M rp ) and the mixing coordinate (ψ mix ). ψ mix describes the radial extent out through the SN ejecta to which the r-process enriched layers are mixed homogeneously (Barnes & Metzger 2022;Anand et al 2024). The limit ψ mix = 1 corresponds to the r-process material being homogeneously mixed all the way to the ejecta surface), resulting in a more pronounced and prolonged red color (Figure 4).…”

“…However, other interpretations, such as interaction with circumstellar material (CSM) or shock cooling, remain plausible explanations for this event (e.g., Ho et al 2020;Izzo et al 2020;Jin et al 2021), and there are no signs of reddening in the SN light curve. Anand et al (2024) performed a comprehensive search for rprocess signatures in collapsar SNe using contemporaneous optical-NIR color measurements for a sample of 25 nearby broad-lined, stripped-envelope SNe (SNe Ic-BL) mostly discovered by the Zwicky Transient Facility (ZTF; including one associated with a GRB, GRB 190829A). SNe Ic-BL are often associated with collapsars (MacFadyen & Woosley 1999), but may be explained with other mechanisms (e.g., Kashiyama et al 2016).…”

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

“…They found that the effect of r-process pollution is to induce reddening in the SN light curves that becomes more prominent with time, as the photosphere retreats inward toward the r-enriched layers. Motivated by this work, efforts to quantify the presence of rprocess elements in stripped-envelope SNe via new or archival infrared follow-up observations have recently begun (Anand et al 2024;Rastinejad et al 2023). Barnes & Metzger (2023) applied their model to interpret the excess infrared emission observed to accompany the nominally long-duration GRB 211211A (Rastinejad et al 2022) to explore whether this event was compatible with being an r-process-enriched SN, rather than a kilonova from a neutron star merger.…”