THE TYPE IIb SUPERNOVA 2013df AND ITS COOL SUPERGIANT PROGENITOR

We have obtained early-time photometry and spectroscopy of Supernova (SN)\n2013df in NGC 4414. The SN is clearly of Type IIb, with notable similarities to\nSN 1993J. From its luminosity at secondary maximum light, it appears that less\n$^{56}$Ni ($\\lesssim 0.06\\ M_{\\odot}$) was synthesized in the SN 2013df\nexplosion than was the case for the SNe IIb 1993J, 2008ax, and 2011dh. Based on\na comparison of the light curves, the SN 2013df progenitor must have been more\nextended in radius prior to explosion than the progenitor of SN 1993J. The\ntotal extinction for SN 2013df is estimated to be $A_V=0.30$ mag. The\nmetallicity at the SN location is likely to be solar. We have conducted Hubble\nSpace Telescope (HST) Target of Opportunity observations of the SN with the\nWide Field Camera 3, and from a precise comparison of these new observations to\narchival HST observations of the host galaxy obtained 14 years prior to\nexplosion, we have identified the progenitor of SN 2013df to be a yellow\nsupergiant, somewhat hotter than a red supergiant progenitor for a normal Type\nII-Plateau SN. From its observed spectral energy distribution, assuming that\nthe light is dominated by one star, the progenitor had effective temperature\n$T_{\\rm eff} = 4250 \\pm 100$ K and a bolometric luminosity $L_{\\rm\nbol}=10^{4.94 \\pm 0.06}\\ L_{\\odot}$. This leads to an effective radius $R_{\\rm\neff} = 545 \\pm 65\\ R_{\\odot}$. The star likely had an initial mass in the range\nof 13 to 17 $M_{\\odot}$; however, if it was a member of an interacting binary\nsystem, detailed modeling of the system is required to estimate this mass more\naccurately. The progenitor star of SN 2013df appears to have been relatively\nsimilar to the progenitor of SN 1993J.\n