SN 2020bio: A Double-peaked, H-poor Type IIb Supernova with Evidence of Circumstellar Interaction

TL;DR

SN 2020bio is a double-peaked Type IIb supernova with unique features suggesting a lower-mass progenitor star that experienced significant mass loss, providing new insights into the diversity of supernova progenitors.

Contribution

This paper presents detailed observations and analysis of SN 2020bio, revealing its peculiar properties and implications for progenitor star characteristics, expanding understanding of Type IIb supernova diversity.

Findings

Progenitor radius estimated at 100–1500 R_sun.

Small amount of synthesized nickel (~0.02 M_sun).

Evidence of greater mass loss than typical Type IIb progenitors.

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, similarly to other well-studied Type IIb SNe. This early-time emission is thought to originate from the cooling of the extended outer hydrogen-rich (H-rich) 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 in order to investigate its progenitor properties. Analytical model fits to the early-time emission give progenitor radius ($\approx$ 100--1500 $R_\odot$) and H-rich envelope mass ($\approx$ 0.01--0.5 $M_\odot$) estimates that are consistent with other Type IIb SNe. However, SN 2020bio displays several peculiarities, including: (1) weak H spectral features indicating a greater amount of mass loss than other Type IIb progenitors; (2) an underluminous secondary light-curve peak that implies a small amount of synthesized $^{56}$Ni ($M_{\text{Ni}}$ $\approx$ 0.02 $M_\odot$); and (3) low-luminosity nebular [O I] and interaction-powered nebular features. These observations are more consistent with a lower-mass progenitor ($M_{\text{ZAMS}} \approx$ 12 $M_\odot$) that was stripped of most of its H-rich envelope before exploding. This study adds to the growing diversity in the observed properties of Type IIb SNe and their progenitors.