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We present a detection in pre-explosion Hubble Space Telescope (HST) imagingof a point source consistent with being the progenitor star of the Type IIbsupernova (SN IIb) 2016gkg. Post-explosion imaging from the Keck AdaptiveOptics system was used to perform relative astrometry between the Keck and HSTimaging. We identify a single point source in the HST images coincident withthe SN position to 0.89-sigma. The HST photometry is consistent with theprogenitor star being an A0Ia star with T=9500 K and log (L/Lsun)=5.15. We findthat the SN 2016gkg progenitor star appears more consistent with binary thansingle-star evolutionary models. In addition, early-time light curve data fromSN 2016gkg revealed a rapid rise in luminosity within ~0.4 days ofnon-detection limits, consistent with models of the cooling phase after shockbreak-out. We use these data to determine an explosion date of 20.15 September2016 and progenitor star radius of log (R/Rsun)=2.41, which agrees withphotometry from the progenitor star. Our findings are also consistent withdetections of other SNe IIb progenitor stars, although more luminous and bluerthan most other examples.

On the Progenitor of the Type IIb Supernova 2016gkg