Red supergiants have been confirmed as the progenitor stars of the majority of hydrogen-rich type II supernovae1. However, while such stars are observed with masses >25 M⊙ (ref. 2), detections of >18 M⊙ progenitors remain elusive1. Red supergiants are also expected to form at all metallicities, but discoveries of explosions from low-metallicity progenitors are scarce. Here, we report observations of the type II supernova, SN 2015bs, for which we infer a progenitor metallicity of ≤0.1 Z⊙ from comparison to photospheric-phase spectral models3, and a zero-age main-sequence mass of 17–25 M⊙ through comparison to nebular-phase spectral models4,5. SN 2015bs displays a normal ‘plateau’ light-curve morphology, and typical spectral properties, implying a red supergiant progenitor. This is the first example of such a high-mass progenitor for a ‘normal’ type II supernova, suggesting a link between high-mass red supergiant explosions and low-metallicity progenitors.
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T.K. and T.-W.C. acknowledge support through the Sofja Kovalevskaja Award provided to P. Schady from the Alexander von Humboldt Foundation of Germany. A.J. acknowledges funding by the European Union's Framework Programme for Research and Innovation Horizon 2020 under Marie Sklodowska-Curie grant agreement no. 702538. S.J.S. acknowledges funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC Grant agreement no.  and Science and Technology Facilities Council (STFC) grants ST/I001123/1 and ST/L000709/1. M.D.S., C.C. and E.Y.H. gratefully acknowledge the generous support provided by the Danish Agency for Science and Technology and Innovation realized through a Sapere Aude Level 2 grant. M.D.S. acknowledges funding by a research grant (13261) from the VILLUM FONDEN. Support for S.G.-G. is provided by the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009 awarded to The Millennium Institute of Astrophysics (MAS), and CONICYT through FONDECYT grant 3140566. Support for C.A. is provided by the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009 awarded to The Millennium Institute of Astrophysics (MAS), and CONICYT through FONDECYT grant 3150463. M.F. acknowledges the support of a Royal Society–Science Foundation Ireland University Research Fellowship. K.M. acknowledges support from the STFC through an Ernest Rutherford Fellowship. M.S. acknowledges support from EU/FP7-ERC grant 615929. The work of the CSP-II has been supported by the National Science Foundation under grants AST0306969, AST0607438, AST1008343 and AST1613426. This work is based (in part) on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile as part of PESSTO (the Public ESO Spectroscopic Survey for Transient Objects) ESO program 188.D-3003, 191.D-0935. This work is based (in part) on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 296.D-5003(A). This work was partly supported by the European Union FP7 programme through ERC grant number 320360. Pan-STARRS is supported by NASA grants NNX08AR22G, NNX14AM74G. PS1 surveys acknowledge the PS1SC: University of Hawaii, MPIA Heidelberg, MPE Garching, Johns Hopkins University, Durham University, University of Edinburgh, Queens University Belfast, Harvard-Smithsonian CfA, LCOGT, NCU Taiwan, STScI, University of Maryland, Eotvos Lorand University, Los Alamos National Laboratory, and NSF grant no. AST-1238877. A. Gal-Yam, M. Bersten, F. Förster, J. Hillier, F. Taddia and C. Fransson are thanked for useful discussions. This research has made use of: the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics; iraf, which is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation; QfitsView; and the SDSS, funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.
Supplementary Figures 1–12, Supplementary Tables 1–5.