The Probable Red Supergiant Progenitor of SN 2017eaw

ATel #10378; Schuyler D. Van Dyk (Caltech/IPAC), Alexei V. Filippenko (UC Berkeley), Ori D. Fox (STScI), Patrick L. Kelly (UC Berkeley), Dan Milisavljevic (Harvard/CfA), and Nathan Smith (Univ. Arizona)
on 15 May 2017; 15:44 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Schuyler D. Van Dyk (vandyk@ipac.caltech.edu)

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We report that we have used the discovery confirmation image of SN 2017eaw in NGC 6946, presented by Dong & Stanek (ATel #10372), to determine the position of the SN site in a publicly available archival Hubble Space Telescope ACS/WFC image at F814W from 2004 July 29, using 12 stars in common between the two images. The accuracy of the determination of the position was compromised by the saturation of the SN in the confirmation image. Nonetheless, we have located a stellar object approximately 6.4 WFC pixels (about 0.3 arcsec) to the southwest of this position. We have measured photometry for this star in the ACS F814W image, as well as in WFC3/IR images at F110W and F128N from 2016 February 9, using Dolphot (Dolphin 2000, PASP, 112, 1397). The star is not detected in an ACS/WFC F658N image, also from 2004 July. We note that no other luminous star is obvious in the ACS and WFC3/IR images out to about 3 arcsec from the nominal position.

Together with the brightness for this star in Spitzer Space Telescope data from 2004 at 3.6 and 4.5 microns reported by Khan (ATel #10373), we find that the overall spectral energy distribution (SED) of the star is roughly consistent with a supergiant stellar atmosphere model at 3500 K (Castelli & Kurucz 2004, ATLAS9), reddened only by the Galactic foreground (A_V = 0.94 mag; Schlafly & Finkbeiner 2011, ApJ, 737, 103; via the NASA/IPAC Extragalactic Database) and assuming a Cardelli et al. (1989, ApJ, 345, 245) reddening law. However, we note that deficiencies in observed flux in the optical and excesses in the infrared, relative to the model, indicate the possible presence of circumstellar dust. The K-band brightness of the model would be approximately 18.7 mag. Assuming the tip-of-the-red-giant-branch distance to the host galaxy, 6.7 Mpc (Tikhonov 2014, AstL, 40, 537), the absolute K magnitude would be -10.5, and further assuming an appropriate bolometric correction at K from Levesque et al. (2005, ApJ, 628, 973), the bolometric magnitude is -7.6 mag, or log (L_bol/L_Sun) ~ 4.9 (assuming M_bol(Sun) = +4.74 mag). This is approximately the locus in the Hertzsprung-Russell diagram of a red supergiant for which the initial mass was about 13 solar masses (from comparison with MIST stellar evolutionary tracks at solar metallicity; Choi et al. 2016, ApJ, 823, 102; Paxton et al. 2011, ApJS, 192, 3; 2013, ApJS, 208, 4; 2015, ApJS, 220, 15), although this estimate is preliminary. Given that the SN has been classified as a Type II-P (ATel #10374, #10376, #10377), we consider it highly likely that this star is the progenitor. However, further confirmation with higher spatial resolution imaging of the SN is required, as is more detailed modeling of the observed SED.