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With the advent of new wide-field, high-cadence optical transient surveys,our understanding of the diversity of core-collapse supernovae has growntremendously in the last decade. However, the pre-supernova evolution ofmassive stars, that sets the physical backdrop to these violent events, istheoretically not well understood and difficult to probe observationally. Herewe report the discovery of the supernova iPTF13dqy = SN 2013fs, a mere ~3 hrafter explosion. Our rapid follow-up observations, which includemultiwavelength photometry and extremely early (beginning at ~6 hrpost-explosion) spectra, map the distribution of material in the immediateenvironment (<~ 10^15 cm) of the exploding star and establish that it wassurrounded by circumstellar material (CSM) that was ejected during the final ~1yr prior to explosion at a high rate, around 10^-3 solar masses per year. Thecomplete disappearance of flash-ionised emission lines within the first severaldays requires that the dense CSM be confined to within <~ 10^15 cm, consistentwith radio non-detections at 70--100 days. The observations indicate thatiPTF13dqy was a regular Type II SN; thus, the finding that the probable redsupergiant (RSG) progenitor of this common explosion ejected material at ahighly elevated rate just prior to its demise suggests that pre-supernovainstabilities may be common among exploding massive stars.

Confined Dense Circumstellar Material Surrounding a Regular Type II Supernova: The Unique Flash-Spectroscopy Event of SN 2013fs