SN 2016iog: A fast declining Type II-L supernova with an ultra-faint tail, persistently interacting with circumstellar material

TL;DR

SN 2016iog is a rapidly declining Type IIL supernova with an ultra-faint tail, showing signs of sustained interaction with circumstellar material, likely from a low-mass hydrogen envelope around a red supergiant.

Contribution

This study provides detailed photometric and spectroscopic observations of a fast-declining Type IIL supernova with unique features indicating ongoing circumstellar interaction.

Findings

SN 2016iog is one of the fastest declining Type IIL SNe observed.

Persistent broad Hα lines indicate sustained interaction with circumstellar material.

Late-time double-peaked Hα suggests dust formation or asymmetric circumstellar environment.

Abstract

We present optical photometric and spectroscopic observations of the rapidly declining Type IIL supernova (SN) 2016iog. SN 2016iog reached its peak $\sim$ 14 days after explosion, with an absolute magnitude in the $V$ band of $-18.64 \pm 0.15$ mag, followed by a steep decline of $8.85 \pm 0.15$~mag~(100\,d)$^{-1}$ post-peak. Such a high decline rate makes SN~2016iog one of the fastest declining Type~IIL SNe observed to date. The rapid rise in the light curve, combined with the nearly featureless continuum observed in the spectrum at +9.3 days, suggests the presence of interaction. In the recombination phase, we observed broad H$\alpha$ lines that persist at all epochs. In addition, the prominent double-peaked H$\alpha$ feature observed in the late-time spectrum (+190.8 days) is likely attributable either to significant dust formation within a cool dense shell or to asymmetric circumstellar material. These features suggest the presence of sustained interaction around SN~2016iog. We propose that the observed characteristics of SN~2016iog can be qualitatively explained by assuming a low-mass H-rich envelope surrounding a red supergiant progenitor star with low-density circumstellar material.