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 $4.5\hat{I}¼$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 $>2\times 10^{-5}{M}_{\odot }$ dust shell with $R=4000R_{\odot }$ and $T=960$ K. Comparing to single-star evolutionary tracks, we find that the progenitor star had an initial mass of $13M_{\odot }$ and a mass-loss rate of $2\times 10^{-7}{M}_{\odot }{\text{yr}}^{-1}$, consistent with the population of SN II progenitor stars.

11 pages, 7 figures, submitted to MNRAS