Fast and not-so-furious: Case study of the fast and faint type IIb SN2021bxu

Dhvanil D Desai, Chris Ashall, Benjamin J Shappee, Nidia Morrell, Lluís Galbany, Christopher R Burns, James M DerKacy, Jason T Hinkle, Eric Hsiao, Sahana Kumar, Jing Lu, Mark M Phillips, Melissa Shahbandeh, Maximilian D Stritzinger, Eddie Baron, Melina C Bersten, Peter J Brown, Thomas de Jaeger, Nancy Elias-Rosa, Gastón FolatelliMark E Huber, Paolo Mazzali, Tomás E Müller-Bravo, Anthony L Piro, Abigail Polin, Nicholas B Suntzeff, Joseph P Anderson, Kenneth C Chambers, Ting-Wan Chen, Thomas de Boer, Michael D Fulton, Hua Gao, Mariusz Gromadzki, Cosimo Inserra, Eugene A Magnier, Matt Nicholl, Fabio Ragosta, Richard Wainscoat, David R Young

Research output: Contribution to journalArticlepeer-review


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 day plateau not caused by H- or He-recombination. SN 2021bxu shows line velocities which are at least ∼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 56Ni. The best-fit parameters for SN 2021bxu include a 56Ni mass of $M_{\mathrm{Ni}} = 0.029^{+0.004}_{-0.005}\, \mathrm{{\rm M}_{\odot }}$, an ejecta mass of $M_{\mathrm{ej}} = 0.61^{+0.06}_{-0.05}\, \mathrm{{\rm M}_{\odot }}$, 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 MNi 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.
Original languageEnglish
Pages (from-to)767–785
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date28 Jun 2023
Publication statusPublished - Sep 2023


  • Space and Planetary Science
  • Astronomy and Astrophysics


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