A potential progenitor for the Type Ic supernova 2017ein

We report the first detection of a credible progenitor system for a Type Ic supernova (SN Ic), SN 2017ein. We present spectra and photometry of the SN, finding it to be similar to carbon-rich, low-luminosity SNe Ic. Using a post-explosion Keck adaptive optics image, we precisely determine the position of SN 2017ein in pre-explosion HST images, finding a single source coincident with the SN position. This source is marginally extended, and is consistent with being a stellar cluster. However, under the assumption that the emission of this source is dominated by a single point source, we perform point-spread function photometry, and correcting for line-of-sight reddening, we find it to have M-F555W = -7.5 +/- 0.2 mag and M-F(555W) - M-F814W= -0.67 +/- 0.14 mag. This source is bluer than the main sequence and brighter than almost all Wolf-Rayet stars, however, it is similar to some WC+O- and B-star binary systems. Under the assumption that the source is dominated by a single star, we find that it had an initial mass of 55(-15)(+20)M(circle dot). We also examined binary star models to look for systems that match the overall photometry of the pre-explosion source and found that the best-fitting model is an 80+48M(circle dot) close binary system in which the 80M(circle dot) star is stripped and explodes as a lower mass star. Late-time photometry after the SN has faded will be necessary to cleanly separate the progenitor star emission from the additional coincident emission.