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Progenitor Candidate for SN 2016gkg in NGC 613

ATel #9536; C. D. Kilpatrick, M. R. Siebert, R. J. Foley (UCSC), C. E. Max (UCO/UCSC), P. Williams, L. E. Abramson, C.-X. Lu, T. Treu (UCLA), M. Kassis (Keck)
on 23 Sep 2016; 17:07 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Charles Kilpatrick (charlesk@email.arizona.edu)

Subjects: Infra-Red, Optical, Star, Supernovae, Transient

Referred to by ATel #: 9556, 9561, 9562, 9573, 9576

We have identified a candidate progenitor star of the Type II SN 2016gkg in NGC 613 (ATel #9521, #9526, #9528, #9529) in archival Hubble Space Telescope (HST) WFPC2 images in the F450W, F606W, and F814W filters. These images were originally obtained on 2001 August 21 UT. We determined the approximate position of the SN in HST images by first measuring a position from the discovery image. This position aligned with a source with magnitudes F450W = 23.17 +/- 0.27 mag, F606W = 23.65 +/- 0.15 mag, and F814W = 24.49 +/- 0.48 mag. The source does not appear to be extended or blended and is consistent with being a single blue star.

We followed up this detection by acquiring K'-band images of SN 2016gkg with the Near-Infrared Camera 2 (NIRC2) on the Keck-II 10-m telescope in conjunction with the adaptive optics system on 2016 Sept 22.49 UT. We corrected these images for astrometric distortion and registered our NIRC2 exposures with sources in the HST archival imaging. The astrometric fit was performed with 6 common sources yielding a root-mean-square scatter of 0.045” in RA and 0.054” in Dec. Comparing to the HST imaging, we find that the position of the SN (RA=01:34:14.421, Dec=-29:26:23.80) is offset from the candidate progenitor star by 0.11”. Taking in to account the astrometric uncertainty and the uncertainty for the position of the HST source (0.06”), our measurements are consistent with the SN and candidate progenitor star having coincident positions (a statistical offset of 1.2 sigma). The identified star remains a viable candidate for the progenitor star of SN 2016gkg.

As noted above, the candidate progenitor star is blue. Removing the Milky Way dust reddening, we find F450W - F606W = -0.50 +/- 0.31 mag. While we have not corrected for host-galaxy reddening, the classification spectrum (Jha, van Wyk, & Vaisanen; ATEL #9528) is also very blue and any additional, host-galaxy reddening requires that the intrinsic color be even bluer. The colors of the candidate progenitor star are consistent with, albeit nominally bluer, than a B3I star (the stellar type of the SN 1987A progenitor star, Sk -69 202; e.g., White & Malin, 1987, Nature, 327, 36; Walborn et al., 1989, A&A, 219, 229). The colors are also consistent with a hot blackbody spectrum (T > 25,000 K). Assuming a Tully-Fisher distance to the host galaxy, NGC 613, of 26.4 Mpc (Nasonova, de Freitas Pacheco, & Karachentsev, 2011, A&A, 532, 104) and no host extinction, the candidate progenitor star has an absolute magnitude of M_F606W = -8.5 +/- 0.4 mag, which would be more luminous than Sk -69 202.

For the F814W image, we measure a limiting magnitude of 24.94 mag. If the identified star is not the progenitor of SN 2016gkg, then the true progenitor star must have M_F814 >~ -7.2 mag, which rules out some, but not all, potential red supergiant progenitor stars.