The Progenitor and Early Evolution of the Type IIb SN 2016gkg

We report initial observations and analysis on the Type IIb SN 2016gkg in the nearby galaxy NGC 613. SN 2016gkg exhibited a clear double-peaked light curve during its early evolution, as evidenced by our intensive photometric follow-up campaign. SN 2016gkg shows strong similarities with other Type IIb SNe, in particular, with respect to the He i emission features observed in both the optical and near-infrared. SN 2016gkg evolved faster than the prototypical Type IIb SN 1993J, with a decline similar to that of SN 2011dh after the first peak. The analysis of archival Hubble Space Telescope images indicate a pre-explosion source at SN 2016gkg’s position, suggesting a progenitor star with a ˜mid-F spectral type and initial mass 15{--}20 M {}⊙ , depending on the distance modulus adopted for NGC 613. Modeling the temperature evolution within 5 {days} of explosion, we obtain a progenitor radius of ˜ 48{--}124 R {}⊙ , smaller than that obtained from the analysis of the pre-explosion images (240{--}320 R {}⊙ ).