Progenitor Candidates for SN 2013df in NGC 4414

ATel #5139; Schuyler D. Van Dyk (IPAC/Caltech), S. Bradley Cenko (NASA/GSFC), Ryan J. Foley (Harvard/Smithsonian CfA), Alexei V. Filippenko (UC Berkeley), Nathan Smith (University of Arizona), and Adam A. Miller (UC Berkeley)
on 17 Jun 2013; 20:52 UT
Credential Certification: Schuyler D. Van Dyk (vandyk@ipac.caltech.edu)

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We have identified three potential candidates for the progenitor star of the Type IIb SN 2013df (CBET 3557; PSN J12262933+3113383), in archival Hubble Space Telescope (HST) WFPC2 images in F439W (~B), F555W (~V), and F814W (~I). These images were originally obtained for the Hubble Heritage project (GO-8400) on 1999 April 29 UT. We isolated the position of the SN in the HST images by first measuring an accurate position from the discovery image, at RA(J2000) = 12h26m29.35s Dec(J2000) = +31d13m37.5s, and then applying an absolute astrometric grid to the Hubble Heritage image mosaics. We also employed a V-band SN image obtained with Swift, which was geometrically transformed to the HST mosaics. Within the 0.75" total positional uncertainty (a 15 drizzled WFPC2-pixel error radius), three stars are seen and also detected by the package Dolphot (Dolphin 2000, PASP, 112, 1383). The brightest has B = 24.24 (+/- 0.12), V = 23.76 (+/- 0.04), and I = 23.34 (+/- 0.06) mag. Assuming the distance modulus to NGC 4414, mu_0 = 31.10 mag (Freedman et al. 2001, ApJ, 553, 47), and only the foreground Galactic extinction, A_V = 0.05 mag from Schlafly & Finkbeiner (2011, ApJ, 737, 103), this results in an extinction-corrected absolute V magnitude of -7.4 and a reddening-corrected color of B-V = 0.46 mag for the star, corresponding approximately to a late-F supergiant. (The host-galaxy extinction is likely modest, based on the Na I D equivalent widths, 0.45 and 0.33 Angstroms, from a Keck-II/DEIMOS spectrum obtained on June 10.) The other two stars are about 0.8-0.9 mag fainter in V and significantly bluer than the brighter star, which we consider the more likely SN progenitor candidate. We have requested activation of a trigger for our HST ToO program (G0-12888; PI: S. D. Van Dyk) to observe the SN and potentially confirm one of the candidates. Moreover, ultraviolet spectra are being obtained with HST/STIS (GO-13030; PI: A. V. Filippenko). Further analysis is ongoing, and observations at all wavelengths are encouraged.