Fast and not-so-furious: Case study of the fast and faint Type IIb SN 2021bxu

We present photometric and spectroscopic observations and analysis of SN 2021bxu (ATLAS21dov), a low-luminosity, fastevolving 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 +/- 0.16 mag during the plateau, it is at the lower end of the luminosity distribution of stripped-envelope supernovae (SE-SNe) and shows a distinct similar to 10 d plateau not caused by H- or He-recombination. SN 2021bxu shows line velocities which are at least similar to 1500 km 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 Ni-56. The best-fitting parameters for SN 2021bxu include a Ni-56 mass of M-Ni = 0.029(-0.005)+M-0.004(circle dot), an ejecta mass of M-ej = 0.61(-0.05)(+0.06)M(circle dot), and an ejecta kinetic energy of K-ej = 8.8(-1.0)(+1.1) x10(49) 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-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.