Luminous Type II Short-Plateau Supernovae 2006Y, 2006ai, and 2016egz: A Transitional Class from Stripped Massive Red Supergiants Hiramatsu, Daichi Galbany González, Lluis Supernovae Core-collapse supernovae Type II supernovae Massive stars Red supergiant stars We are grateful to Viktoriya Morozova, Anthony L. Piro, Lars Bildsten, Bill Paxton, J. J. Eldridge, Eva Laplace, Sergei I. Blinnikov, Ken'ichi Nomoto, Masaomi Tanaka, and Nozomu Tominaga for their comments and discussions. D.H., D.A.H., G.H., C.M., and J.B. were supported by NSF grants AST-1313484 and AST-1911225, as well as by NASA grant 80NSSC19kf1639. D.H. is thankful for support and hospitality by the National Astronomical Observatory of Japan where many discussions of this work took place. J.A.G. is supported by the NSF GRFP under grant number 1650114. I.A. is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that program, from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement number 852097), from the Israel Science Foundation (grant numbers 2108/18 and 2752/19), from the United States - Israel Binational Science Foundation (BSF), and from the Israeli Council for Higher Education Alon Fellowship. C.P.G acknowledges support from EU/FP7- ERC grant number 615929. Research by S.V. is supported by NSF grant AST{1813176. Q.F. acknowledges support from the JSPS Kakenhi grant (20J23342). K. Maeda acknowledges support by JSPS KAKENHI grants JP20H00174, JP20H04737, JP18H04585, JP18H05223, and JP17H02864. G.F. acknowledges the support from CONICET through grant PIP-2015-2017- 11220150100746CO and from ANPCyT through grant PICT-2017-3133. M.D.S. acknowledges support from a project grant (8021-00170B) from the Independent Research Fund Denmark and a generous grant (28021) from the VILLUM FONDEN. M.G. is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. K. Maguire is funded by the EU H2020 ERC grant number 758638. T.M.B. was funded by the CONICYT PFCHA/DOCTORADOBECAS CHILE/2017- 72180113. This paper made use of data from the Las Cumbres Observatory global network of telescopes through the Supernova Key Project and Global Supernova Project. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. The CSP-I was supported by the NSF under grants AST-0306969, AST-0607438 and AST-1008343. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by NASA and operated by the California Institute of Technology, as well as IRAF, which is distributed by NOAO (operated by AURA, Inc.), under cooperative agreement with NSF. The authors wish to recognize and acknowledge the very signi cant cultural role and reverence that the summit of Haleakal a has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from the mountain. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy O ce of Science. The SDSS-III website is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration, including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astro sica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor's hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus (~100 days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about 50–70 days and luminous optical peaks (lesssim−18.4 mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of MESA+STELLA single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies, 56Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP–IIL–IIb-like light-curve morphology in descending order of H-rich envelope mass. With large 56Ni masses (gsim0.05 M⊙), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors (MZAMS sime 18–22 M⊙) with small H-rich envelope masses (${M}_{{{\rm{H}}}_{\mathrm{env}}}\simeq 1.7\,{M}_{\odot }$) that have experienced enhanced mass loss ($\dot{M}\simeq {10}^{-2}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples. 2021-07-01T10:12:37Z 2021-07-01T10:12:37Z 2021-05-27 info:eu-repo/semantics/article Publisher version: Daichi Hiramatsu et al. 2021 ApJ 913 55. [https://doi.org/10.3847/1538-4357/abf6d6] http://hdl.handle.net/10481/69454 10.3847/1538-4357/abf6d6 eng info:eu-repo/grantAgreement/EC/H2020/852097 info:eu-repo/grantAgreement/EU/FP7- ERC/615929 http://creativecommons.org/licenses/by-nc-nd/3.0/es/ info:eu-repo/semantics/openAccess Atribución-NoComercial-SinDerivadas 3.0 España IOP Publishing