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

One of the open questions following the discovery of GW170817 is whether neutron star mergers are the primary astrophysical sites capable of producing r-process elements. Simulations have shown that r-process elements could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both neutron star mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of r-process nucleosynthesis in the binary neutron star merger GW170817 was its long-lasting near-infrared emission, thus motivating a systematic near-infrared study of the lightcurves of broadlined stripped-envelope core-collapse (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL — including 18 SNe discovered with the Zwicky Transient Facility and 7 from the literature — in the optical and near-infrared 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 and derive constraints on the r-process mass and the mixing parameter for each SN. We find that only one object from our sample of 25, SN 2007I shows unusual photometric reddening relative to the remainder of the sample. However, with independent light curve fits to both r-process enriched and r-process free models, we find no strong evidence of r-process enrichment in any of our objects; all objects, including SN 2007I can be fit with an Arnett model. Further high cadence infrared photometric studies, nebular spectroscopic analysis, and multi-wavelength observations will shed light on whether all collapsars are devoid of r-process nucleosynthesis, or whether only specific types of collapsars can synthesize heavy elements.