SN 2017ens: The Metamorphosis of a Luminous Broad-lined Type Ic Supernova into an SN IIn

TL;DR

SN 2017ens exhibited a unique spectral evolution from a luminous broad-lined Type Ic supernova to showing features of an SN IIn, indicating complex circumstellar interactions and possible progenitor mass-loss history.

Contribution

This study documents the first detailed observation of a supernova transforming from a broad-lined Type Ic to an SN IIn, revealing insights into progenitor mass loss and circumstellar environment.

Findings

Spectral evolution from blue, featureless to broad-lined Type Ic features.

Persistent narrow Balmer emission indicating dense circumstellar medium.

Detection of coronal lines suggesting high-density environment.

Abstract

We present observations of supernova (SN) 2017ens, discovered by the ATLAS survey and identified as a hot blue object through the GREAT program. The redshift z=0.1086 implies a peak brightness of M_g=-21.1 mag, placing the object within the regime of superluminous supernovae. We observe a dramatic spectral evolution, from initially being blue and featureless, to later developing features similar to those of the broadlined Type Ic SN 1998bw, and finally showing ~2000 km s^-1 wide H-alpha and H-beta emission. Relatively narrow Balmer emission (reminiscent of a SN IIn) is present at all times. We also detect coronal lines, indicative of a dense circumstellar medium. We constrain the progenitor wind velocity to ~50-60 km s^-1 based on P-Cygni profiles, which is far slower than those present in Wolf-Rayet stars. This may suggest that the progenitor passed through a luminous blue variable phase, or that the wind is instead from a binary companion red supergiant star. At late times we see the ~2000 km s^-1 wide H-alpha emission persisting at high luminosity (~3x10^40 erg s^-1) for at least 100 day, perhaps indicative of additional mass loss at high velocities that could have been ejected by a pulsational pair instability.