Photospheric velocity evolution of SN 2020bvc: signature of $r$-process nucleosynthesis from a collapsar

Abstract

Whether binary neutron star mergers are the only astrophysical site of rapid neutron-capture process ($r$-process) nucleosynthesis remains unknown. Collapsars associated with long gamma-ray bursts (GRBs) and hypernovae are promising candidates. Simulations have shown that outflows from collapsar accretion disks can produce enough $r$-process materials to explain the abundances in the universe. However, there is no observational evidence to confirm this result at present. SN 2020bvc is a broad-lined type Ic (Ic-BL) supernova (SN) possibly associated with a low-luminosity GRB. Based on semi-analytic SN emission models with and without $r$-process materials, we perform a fitting to the multi-band light curves and photospheric velocities of SN 2020bvc. We find that in a $r$-process-enriched model the mixing of $r$-process materials slows down the photospheric recession and therefore matches the velocity evolution better. The fitting results show that $r$-process materials with mass of $\approx0.36~M_\odot$ and opacity of $\approx4~\rm cm^2~g^{-1}$ is needed to mix with about half of the SN ejecta. Our fitting results are weakly dependent on the nebular emission. Future statistical analysis of a sample of type Ic-BL SNe helps us understand the contribution of collapsars to the $r$-process abundance.