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 $\sim1500\,\mathrm{km\,s<sup>{-1}}$ 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 $<sup>{56}$Ni. The best-fit parameters for SN 2021bxu include a $<sup>{56}$Ni mass of $$M_{\mathrm{Ni}} = 0.029<sup>{+0.004}<em>{-0.005}\,\mathrm{M</em>{\odot}}$$, an ejecta mass of $$M_{\mathrm{ej}} = 0.61<sup>{+0.06}<em>{-0.05}\,\mathrm{M</em>{\odot}}$$, and an ejecta kinetic energy of $$K_{\mathrm{ej}} = 8.8^{{+1.1}_{-1.0}} \times 10^{49}\, \mathrm{erg}$$. 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.