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 yr to 43 d before discovery. While the progenitor luminosity was roughly constant for most of this period, there was a ∼20 per cent increase in its 4.5 μm luminosity over the final 3 yr 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_⊙) = 4.9 and T = 3350 K, obscured by a >2 × 10^-5 M_⊙ dust shell with R = 4000 R_{⊙} and T = 960 K. Comparing to single-star evolutionary tracks, we find that the progenitor star had an initial mass of 13 M_{⊙} and a mass-loss rate of 2× 10^{-7} M_{⊙} yr^{-1}, consistent with the population of SN II progenitor stars.