Wang, QinanArmstrong, PatrickZenati, YossefRidden-Harper, RyanRest, ArminArcavi, IairKilpatrick, Charles D.Foley, Ryan J.Tucker, Brad E.Lidman, ChrisKillestein, Thomas L.Shahbandeh, MelissaAnderson, Joseph P.Angulo, RodrigoAshall, ChrisBurke, JamisonChen, Ting-Wanvon Coelln, SophieDalrymple, Kyle A.Davis, Kyle W.Fulton, Michael D.Galbany, LluisGonzalez, Estefania PadillaGao, BoreGromadzki, MariuszHowell, D. AndrewIhanec, NadaJencson, Jacob E.Jones, David O.Lyman, Joseph D.McCully, CurtisMuller-Bravo, Tomas E.Newsome, MeganNicholl, MattO'Neill, DavidPellegrino, CraigRest, SofiaSmartt, Stephen J.Smith, KenSrivastav, ShubhamTerreran, GiacomoTinyanont, SamapornYoung, David R.Zenteno, Alfredo2023-03-272023-03-272023-02-01L15http://hdl.handle.net/10919/114190We present early observations and analysis of the double-peaked Type IIb supernova (SN IIb) SN 2021zby. TESS captured the prominent early shock-cooling peak of SN 2021zby within the first similar to 10 days after explosion with a 30 minute cadence. We present optical and near-infrared spectral series of SN 2021zby, including three spectra during the shock-cooling phase. Using a multiband model fit, we find that the inferred properties of its progenitor are consistent with a red supergiant or yellow supergiant, with an envelope mass of similar to 0.30-0.65 M-circle dot and an envelope radius of similar to 120-300 R-circle dot. These inferred progenitor properties are similar to those of other SNe IIb with a double-peaked feature, such as SNe 1993J, 2011dh, 2016gkg, and 2017jgh. This study further validates the importance of the high cadence and early coverage in resolving the shape of the shock-cooling light curve, while the multiband observations, particularly UV, are also necessary to fully constrain the progenitor properties.application/pdfenCreative Commons Attribution 4.0 InternationalYellow supergiant progenitorwolf-rayet starsiib supernovaearly evolutionia supernovamass-loss2011dhemissionspectraspectrographArticle - Refereedhttps://doi.org/10.3847/2041-8213/acb0d094322041-8213