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 Mr=-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 ~10 d plateau not caused by H- or He-recombination. SN 2021bxu shows line velocities which are at least ~1500kms 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 ⁵⁶Ni. The best-fitting parameters for SN 2021bxu include a ⁵⁶Ni mass of M_Ni = 0.61 ^+0.05_-0.06 M_⊙, an ejecta mass of M_ej = 0.61^+0.06_-0.05 M_⊙ and an ejecta kinetic energy of K_ej = 8.8^+1.1_-1.0 X 10⁴⁹ 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.

https://ui.adsabs.harvard.edu/abs/2023MNRAS.524..767D/abstract