With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, which sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF 13dqy = SN 2013fs a mere ∼3 h after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ∼6 h post-explosion) spectra, map the distribution of material in the immediate environment (≲1015 cm) of the exploding star and establish that it was surrounded by circumstellar material (CSM) that was ejected during the final ∼1 yr prior to explosion at a high rate, around 10−3 solar masses per year. The complete disappearance of flash-ionized emission lines within the first several days requires that the dense CSM be confined to within ≲1015 cm, consistent with radio non-detections at 70–100 days. The observations indicate that iPTF 13dqy was a regular type II supernova; thus, the finding that the probable red supergiant progenitor of this common explosion ejected material at a highly elevated rate just prior to its demise suggests that pre-supernova instabilities may be common among exploding massive stars.
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We are grateful to the staff at the various observatories where data were obtained, as well as to N. E. Groeneboom, K. I. Clubb, M. L. Graham, D. Sand, A. A. Djupvik, I. Shivvers, J. C. Mauerhan and A. Waszczak for assistance with observations. We thank E. Waxman for valuable discussions. A.G.-Y.’s group is supported by the EU/FP7 via an ERC grant, the Quantum Universe I-Core programme by the Israeli Committee for planning and budgeting and the ISF; by Minerva and ISF grants; by the Weizmann-UK ‘making connections’ programme; and by Kimmel, ARCHES and Yes awards. D.A.P. acknowledges support from Hubble Fellowship grant HST-HF-51296.01-A awarded by the Space Telescope Science Institute, and from a Marie Curie Individual Fellowship as part of the Horizon 2020 European Union (EU) Framework Programme for Research and Innovation (H2020-MSCA-IF-2014-660113). J.H.G. acknowledges support from an AMBIZIONE grant of the Swiss NSF. E.O.O. is supported by the Arye Dissentshik career development chair, Israel Science Foundation, Minerva, Weizmann-UK, and the I-Core programme. M.M.K. acknowledges support from the National Science Foundation for the GROWTH project funded under Grant No. 1545949. A.V.F.’s research is supported by the Christopher R. Redlich Fund, the TABASGO Foundation, and US NSF grant AST-1211916. Support for I.A. was provided by NASA through the Einstein Fellowship Program, grant PF6-170148. LANL participation in iPTF is supported by the US Department of Energy as part of the Laboratory Directed Research and Development programme. Supernova research at the Oskar Klein Centre is supported by the Swedish Research Council and by the Knut and Alice Wallenberg Foundation. K.M. acknowledges support from a Marie Curie Intra-European Fellowship, within the 7th European Community Framework Programme (FP7). Some data were obtained with the Nordic Optical Telescope, which is operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain. We thank the RATIR project team, the staff of the Observatorio Astronomico Nacional on Sierra San Pedro Martir, and the software support team from Teledyne Scientific and Imaging. RATIR, the automation of the Harold L. Johnson Telescope of the Observatorio Astronomico Nacional and the operation of both are funded through National Aeronautics and Space Administration (NASA) grants NNX09AH71G, NNX09AT02G, NNX10AI27G and NNX12AE66G, CONACyT (INFR-2009-01-122785, CB-2008-101958), UNAM PAPIIT (IN113810 and IG100414) and UCMEXUS-CONACyT. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. A portion of this work was carried out at the Jet Propulsion Laboratory under a Research and Technology Development Grant, under contract with NASA.