2005IAUC.8556....1L

W. Li, University of California at Berkeley; S. D. Van Dyk, Spitzer Science Center, California Institute of Technology; and A. V. Filippenko, University of California at Berkeley (UCB), report that they have isolated a possible progenitor for this young type-II supernova in Hubble Space Telescope images of M51 (GO/DD program 10452) taken with the Advanced Camera for Surveys (ACS). A geometrical transformation between recent KAIT images of the supernova and the ACS images has yielded the identification of an object (mag 24.0 ± 0.2) in the ACS F814W images that is within the 0".15 uncertainty of the supernova position; this object was not detected in the F555W images (mag > 25.5). Assuming low reddening to the supernova [E(B-V) = 0.1 mag; cf. IAUC 8555] and a true distance modulus to M51 of 29.6 mag (Richmond et al. 1996, A.J. 111, 327), the object is consistent with a red supergiant with an approximate absolute I magnitude of -5.8 and colors V-I > +1.4, implying an initial mass for the progenitor of < about 10 solar masses. The supernova occurred in a crowded field, and there are several other red sources and a bright, blue, possible compact star cluster within 0".3 of the supernova location. High-resolution images of the supernova and a more thorough analysis are required to verify the identification of the progenitor.