SN 2020bio: A Double-peaked Type IIb Supernova with Evidence of Early-time Circumstellar Interaction

Abstract

We present photometric and spectroscopic observations of SN 2020bio, a double-peaked Type IIb supernova (SN) discovered within a day of explosion, primarily obtained by Las Cumbres Observatory and Swift. SN 2020bio displays a rapid and long-lasting initial decline throughout the first week of its light curve, similar to other well-studied Type IIb SNe. This early-time emission is thought to originate from the cooling of the extended outer envelope of the progenitor star that is shock-heated by the SN explosion. We compare SN 2020bio to a sample of other double-peaked Type IIb SNe to investigate its progenitor properties. Analytical model fits to the early-time emission give progenitor radius (≈ 100-1500 R ) and H-rich envelope mass (≈ 0.01-0.5 M ) estimates that are consistent with other Type IIb SNe. However, SN 2020bio displays several peculiarities, including: 1) weak H spectral features and narrow emission lines indicative of pre-existing circumstellar material; 2) an underluminous secondary light curve peak which implies a small amount of synthesized 56 Ni (M Ni ≈ 0.02 M ); and 3) lowluminosity nebular [O I] features. These observations are more consistent with a lower-mass progenitor (M ZAMS ≈ 12 M ) that was stripped of most of its H envelope before exploding. This study adds to the growing diversity in the observed properties of Type IIb SNe and their progenitors.