tokens: we find τ m ≈ 12 days, and we obtain a similar value for SN 2017ein. Therefore, the different M ej values inferred for SN 2007gr and SN 2017ein are mainly a result of different opacity values adopted in the analysis. Moreover, the discrepancy in the ejected mass could also be related to the fitting methods and wave bands of the light curve used in the analysis. SN 2017ein is one of the two SNe Ic showing possible evidence for early shock breakout cooling. Another candidate is LSQ14efd, with its V -band light curve showing a bump feature at early times, which was also interpreted as cooling of the shocked stellar envelope (Barbarino et al. 2017 ). In comparison with SN 2017ein, LSQ14efd is more luminous and energetic, with an R -band peak absolute magnitude of −18.14 mag, kinematic energy E kin = 5.3 B, and ejecta mass M ej = 6.3 M ⊙ , making it stand between normal SNe Ic and SNe Ic-BL. 7. <italic>HST</italic> Progenitor Observation 7.1. Pre- and Post-explosion <italic>HST</italic> Images and Data Reduction We searched the pre-explosion HST images from the Mikulski Archive for Space Telescopes (MAST) 29 29 http://archive.stsci.edu/ and the Hubble Legacy Archive 30 30 http://hla.stsci.edu/ and found publicly available F555W and F814W WFPC2 images taken on 2007 December 11 (PI: W. Li). An F438W-band image at the location of SN 2017ein was taken by HST on 2017 June 12 with the WFC3/UVIS configuration (PI: van Dyk). This image was released and publicly available immediately after the observation. We use this image as a post-explosion image to identify the position of the SN progenitor. Table 8 lists the information of these images. Table 8 Pre- and Post-explosion HST Images on the Site of SN 2017ein The pre- and post-discovery HST images around the SN position are shown in Figure 16 , where the zoomed-in regions centered at the SN site are also shown. There is clearly a point-like source near the SN position in both the F555W and F814W images. The position of this point-like source is obtained by averaging the measured values with four different methods (centroid, Gauss, o -filter center algorithm of IRAF task phot, and IRAF task Daophot). On all pre-explosion images, the separation between the position transformed from the post-explosion SN site and the center of the nearby point-like source is within the uncertainty of the transformed position, suggesting that this nearby point source is the progenitor of SN 2017ein. The coordinates of the SN on the post-explosion F438 image are found to be (539.302 ± 0.002, 564.516 ± 0.003) with different methods (centroid, Gauss and o -filter center algorithm of IRAF task phot, IRAF task imexamine, and SExtractor) and are converted to (798.70, 1766.77) and (798.38, 1766.67) on the pre-explosion F555W and F814W images, respectively, as the coordinates of the progenitor. Figure 16. Top: regions around SN 2017ein showing the prediscovery HST F555W (left) and F814W (middle) images and the post-explosion HST F438W
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