* Astronomy

Title: Broad-Lined Supernova 2016coi with Helium Envelope
Author: Masayuki Yamanaka, Tatsuya Nakaoka, Masaomi Tanaka, Keiichi Maeda, Satoshi Honda, Hidekazu Hanayama, Tomoki Morokuma, Masataka Imai, Kenzo Kinugasa, Katsuhiro L. Murata, Takefumi Nishimori, Osamu Hashimoto, Hirotaka Gima, Kensuke Hosoya, Ayano Ito, Mayu Karita, Miho Kawabata, Kumiko Morihana, Yuto Morikawa, Kotone Murakami, Takahiro Nagayama, Tatsuharu Ono, Hiroki Onozato, Yuki Sarugaku, Mitsuteru Sato, Daisuke Suzuki, Jun Takahashi, Masaki Takayama, Hijiri Yaguchi, Hiroshi Akitaya, Yuichiro Asakura, Koji S. Kawabata, Daisuke Kuroda, Daisaku Nogami, Yumiko Oasa, Toshihiro Omodaka, Yoshihiko Saito, Kazuhiro Sekiguchi, Nozomu Tominaga, Makoto Uemura, Makoto Watanabe

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} 5876, 6678, and 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.

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