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Further Classification of SN 2016gkg as a Probable Type IIb Supernova

ATel #9573; Schuyler D. Van Dyk (IPAC/Caltech), WeiKang Zheng, Isaac Shivvers, Alexei V. Filippenko (UC Berkeley), Brad E. Tucker (Mount Stromlo Obs., Australian National University), Daniel A. Perley (Dark Cosmology Centre, NBI), Nathan Smith (U. Arizona)
on 30 Sep 2016; 05:51 UT
Credential Certification: Schuyler D. Van Dyk (vandyk@ipac.caltech.edu)

Subjects: Optical, Supernovae

Referred to by ATel #: 9576

We obtained a spectrum of SN 2016gkg (ATel #9521, #9526, #9528, #9529, #9536, #9544, #9556, #9561, #9562) with the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995, PASP, 107, 375) on the Keck-I 10-m telescope on 2016 September 28.56 UT under good sky conditions. The spectral range was 3160-10270 Ang. We find good matches, using SNID (Blondin & Tonry 2007, ApJ, 666, 1024), with several Type IIb SNe, including SNe 1993J, 2008aq, 2008bo, and 2008cw, within a few days of secondary maximum light. We can make an especially good by-eye comparison with the spectrum of SN 1993J obtained 2 days before secondary maximum (Matheson et al. 2000, AJ, 120, 1487). We find less overall agreement with early-time spectra of the Type II SN 1987A (Pun et al. 1995, ApJS, 99, 223; obtained from WISeREP, Yaron & Gal-Yam 2012, PASP, 124, 668). Based on the similarity with Type IIb supernovae, we predict that the He I 6678 Ang, 7065 Ang, and 7281 Ang absorption lines should start to appear soon in spectra of SN 2016gkg (we are unable to determine whether the feature in our spectrum near 5880 Ang is He I or Na I D). We encourage further monitoring of this supernova at all wavelengths.