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We present a moderate-resolution spectrum of the peculiar Type II supernovaiPTF14hls taken on day 1153 after discovery. This spectrum reveals the clearsignature of shock interaction with dense circumstellar material (CSM). Wesuggest that this CSM interaction may be an important clue for understandingthe extremely unusual photometric and spectroscopic evolution seen over thefirst 600 days of iPTF14hls. The late-time spectrum shows a double-peakedintermediate-width H-alpha line indicative of expansion speeds around 1000km/s, with the double-peaked shape hinting at a disc-like geometry in the CSM.If the CSM was highly asymmetric, perhaps in a disc or torus that was ejectedfrom the star 3-6 years prior to explosion, then the CSM interaction could havebeen overrun and hidden below the SN ejecta photosphere from a wide range ofviewing angles. In that case, CSM interaction luminosity would have beenthermalized well below the photosphere, possibly sustaining the high luminositywithout exhibiting the traditional observational signatures of strong CSMinteraction (narrow H-alpha emission and X-rays). Variations in densitystructure of the CSM could account for the multiple rebrightenings of thelightcurve. We propose that enveloped CSM interaction as seen in some recentSNe, rather than an entirely new explosion mechanism, may be adequate toexplain the peculiar evolution of iPTF14hls.

Strong late-time circumstellar interaction in the peculiar supernova iPTF14hls