Strong late-time circumstellar interaction in the peculiar supernova iPTF14hls

TL;DR

This paper presents evidence of late-time shock interaction with dense circumstellar material in supernova iPTF14hls, suggesting asymmetric CSM may explain its unusual brightness and spectral evolution over 600 days.

Contribution

It introduces the idea that enveloped CSM interaction, possibly hidden beneath the ejecta, explains iPTF14hls's peculiar features without requiring new explosion mechanisms.

Findings

Detection of double-peaked H-alpha line indicating disc-like CSM geometry

Evidence of shock interaction with dense CSM at day 1153

CSM interaction may account for multiple lightcurve rebrightenings

Abstract

We present a moderate-resolution spectrum of the peculiar Type II supernova iPTF14hls taken on day 1153 after discovery. This spectrum reveals the clear signature of shock interaction with dense circumstellar material (CSM). We suggest that this CSM interaction may be an important clue for understanding the extremely unusual photometric and spectroscopic evolution seen over the first 600 days of iPTF14hls. The late-time spectrum shows a double-peaked intermediate-width H-alpha line indicative of expansion speeds around 1000 km/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 ejected from the star 3-6 years prior to explosion, then the CSM interaction could have been overrun and hidden below the SN ejecta photosphere from a wide range of viewing angles. In that case, CSM interaction luminosity would have been thermalized well below the photosphere, possibly sustaining the high luminosity without exhibiting the traditional observational signatures of strong CSM interaction (narrow H-alpha emission and X-rays). Variations in density structure of the CSM could account for the multiple rebrightenings of the lightcurve. We propose that enveloped CSM interaction as seen in some recent SNe, rather than an entirely new explosion mechanism, may be adequate to explain the peculiar evolution of iPTF14hls.