Fast and Not-so-Furious: Case Study of the Fast and Faint Type IIb SN 2021bxu

Abstract

We present photometric and spectroscopic observations and analysis of SN 2021bxu (ATLAS21dov), a low-luminosity, fast-evolving Type IIb supernova (SN). SN 2021bxu is unique, showing a large initial decline in brightness followed by a short plateau phase. With $M_r=-15.93\pm 0.16\mathrm{mag}$ during the plateau, it is at the lower end of the luminosity distribution of stripped-envelope supernovae (SE-SNe) and shows a distinct $\sim$10 day plateau not caused by H- or He-recombination. SN 2021bxu shows line velocities which are at least $\sim 1500\mathrm{km}{\mathrm{s}}^{-1}$ slower than typical SE-SNe. It is photometrically and spectroscopically similar to Type IIb SNe during the photospheric phases of evolution, with similarities to Ca-rich IIb SNe. We find that the bolometric light curve is best described by a composite model of shock interaction between the ejecta and an envelope of extended material, combined with a typical SN IIb powered by the radioactive decay of ${}^{56}$Ni. The best-fit parameters for SN 2021bxu include a ${}^{56}$Ni mass of $M_{\mathrm{Ni}}=0.029_{-0.005}^{+0.004}{\mathrm{M}}_{\odot }$, an ejecta mass of $M_{\mathrm{ej}}=0.61_{-0.05}^{+0.06}{\mathrm{M}}_{\odot }$, and an ejecta kinetic energy of $K_{\mathrm{ej}}=8.8_{-1.0}^{+1.1}\times 10^{49}\mathrm{erg}$. From the fits to the properties of the extended material of Ca-rich IIb SNe we find a trend of decreasing envelope radius with increasing envelope mass. SN 2021bxu has $M_{\mathrm{Ni}}$ on the low end compared to SE-SNe and Ca-rich SNe in the literature, demonstrating that SN 2021bxu-like events are rare explosions in extreme areas of parameter space. The progenitor of SN 2021bxu is likely a low mass He star with an extended envelope.

18 pages, 15 figures, accepted to MNRAS