# Broad-Lined Supernova 2016coi with Helium Envelope

## TL;DR

This study analyzes SN 2016coi, revealing it has helium features and properties intermediate between broad-lined Type Ic and Ib supernovae, suggesting a new classification and highlighting diversity in supernova outer layers.

## Contribution

It demonstrates that SN 2016coi exhibits helium features and properties bridging SNe Ic-BL and Ib, proposing a new classification as broad-lined Type 'Ib'-BL supernovae.

## Key findings

- SN 2016coi shows helium absorption lines up to 12 days post-explosion.
- Line velocities and light curve analysis align SN 2016coi with SNe Ic-BL.
- SN 2016coi and similar SNe may be transitional objects between SNe Ic-BL and Ib.

## Abstract

We present the early-phase spectra and the light curves of the broad-lined supernova (SN) 2016coi from $t=7$ to $67$ days after the estimated explosion date. This SN was initially reported as a broad-lined Type SN Ic (SN Ic-BL). However, we found that spectra up to $t=12$ days exhibited the He~{\sc i} $\lambda$5876, $\lambda$6678, and $\lambda$7065 absorption lines. We show that the smoothed and blueshifted spectra of normal SNe Ib are remarkably similar to the observed spectrum of SN 2016coi. The line velocities of SN 2016coi were similar to those of SNe Ic-BL and significantly faster than those of SNe Ib. Analyses of the line velocity and light curve suggest that the kinetic energy and the total ejecta mass of SN 2016coi are similar to those of SNe Ic-BL. Together with broad-lined SNe 2009bb and 2012ap for which the detection of He~{\sc i} were also reported, these SNe could be transitional objects between SNe Ic-BL and SNe Ib, and be classified as broad-lined Type `Ib' SNe (SNe `Ib'-BL). Our work demonstrates the diversity of the outermost layer in broad-lined SNe, which should be related to the variety of the evolutionary paths.