Early-Time Spitzer Observations of the Type II Plateau Supernova SN 2004dj

We present mid-infrared observations with the Spitzer Space Telescope of the nearby Type II-P supernova SN 2004dj at epochs of 89-129 days. We have obtained the first mid-IR spectra of any supernova apart from SN 1987A. A prominent [Ni II] 6.64 μm line is observed, from which we deduce that the mass of stable nickel must be at least 2.2×10^-4 M_solar. We also observe the red wing of the CO fundamental band. We relate our findings to possible progenitors and favor an evolved star, most likely a red supergiant, with a probable initial mass between ~10 and 15 M_solar.