The Dusty Progenitor Star of the Type II Supernova 2017eaw

Abstract: We present pre-explosion photometry of the likely progenitor star of the Type II supernova (SN II) 2017eaw in NGC 6946. We use a Hubble Space Telescope (HST) image of SN 2017eaw to perform relative astrometry with HST and Spitzer Space Telescope (Spitzer) imaging, finding a single point source consistent with its position. We detect the progenitor star in $>$40 epochs of HST and Spitzer imaging covering 12.9 years to 43 days before discovery. While the progenitor luminosity was roughly constant for most of this period, there was a $\sim$20% increase in its $4.5~\mu$m luminosity over the final 3 years before explosion. We interpret the bright mid-infrared emission as a signature of circumstellar dust around the progenitor system. Using the pre-explosion photometry and assuming some circumstellar dust, we find the progenitor is most likely a red supergiant with $\log(L/L_{\odot}) = 4.9$ and $T = 3350$ K, obscured by a $>2\times10^{-5}~M_{\odot}$ dust shell with $R = 4000~R_{\odot}$ and $T = 960$ K. Comparing to single-star evolutionary tracks, we find that the progenitor star had an initial mass of $13~M_{\odot}$ and a mass-loss rate of $2\times10^{-7}~M_{\odot}~\text{yr}^{-1}$, consistent with the population of SN II progenitor stars.