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Constraints on the Progenitor of SN 2013ai (=PSN J06161835-2122329) in NGC 2207

ATel #4862; Dan Milisavljevic, Alicia Soderberg, Ryan Foley, Ryan Chornock, Wen-fei Fong, Peter Williams, Edo Berger, Maria Drout, Raffaella Margutti, (Harvard University); and Schuyler D. Van Dyk (Spitzer Science Center/Caltech)
on 6 Mar 2013; 05:06 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: D. Milisavljevic (dmilisav@cfa.harvard.edu)

Subjects: Optical, Ultra-Violet, Supernovae

Referred to by ATel #: 4901

We report preliminary analysis of archival Hubble Space Telescope (HST) images in an attempt to identify the progenitor star of the recently discovered and confirmed Type II supernova SN 2013ai (= PSN J06161835-2122329; CBET 3431, ATel #4849, ATel #4851).

We retrieved archival HST images of the field surrounding the SN from the Hubble Legacy Archive and the Mikulski Archive for Space Telescopes. Images were originally obtained with the Wide Field Planetary Camera 2 (WFPC2) with filters F336W, F439W, F555W, and F814W on 1996-05-25 under program GO 6483 (PI: D. Elmegreen).

Two short r-band exposures of the SN were obtained with the 4m Blanco Telescope at Cerro Tololo Inter-American Observatory using the Dark Energy Camera (DECam) on 2013-03-04 UT (during program 2013A-0214; PI Berger) under 1" seeing conditions. We astrometrically registered the images using five fiducial stars common to both HST and DECam images and the IRAF/PYRAF software applications geomap and geotran. The alignment was good to 1-sigma errors of 0.75 pixel (0.1 arcsec) in x (east-west) and 2.0 pixel (0.2 arcsec) in y (north-south).

A relatively bright, blue source is located close to the SN, and we used the package Dolphot v2.0 (Dolphin 2000, PASP, 112, 1383) to conduct photometry on the available images. Assuming a distance to the host galaxy NGC 2207 of 26.5 Mpc and Galactic extinction of A_v = 0.237 mag (retrieved from NED), the absolute extinction-corrected magnitudes of the blue source are approximately M_U = -9.6 mag, M_B = -8.5 mag, M_V = -9.0 mag, and M_I = -9.7 mag. This source is likely a tight stellar cluster, but the data do not rule out a single, massive, luminous progenitor.

We note that within the 3-sigma error ellipse is a much fainter red source that is weakly visible in the F555W image and clearly detected in the F814W image. It lies 0.24 arcsec west and 0.54 arcsec north of the blue source. The estimated extinction-corrected absolute magnitude is M_I = -7.9 mag, which places it in the plausible range of a red supergiant.

Multi-wavelength follow-up observations of this SN are in progress, and further studies of the progenitor are ongoing.

Aligned DECam and HST images of SN 2013ai and its candidate progenitor system