2005IAUC.8633....1P

J. Prieto, Ohio State University (OSU); P. Garnavich, University of Notre Dame; and D. DePoy, J. Marshall, J. Eastman, and S. Frank, OSU, on behalf of the SDSS-II Supernova Collaboration, report that spectra (range 390-730 nm; resolution 0.31 nm/pixel) of SN 2005gj (cf. CBET 247, IAUC 8616) were obtained with the MDM 2.4-m telescope (+ CCDS) approximately once per week since its discovery. Initially classified as a type-IIn event based on the presence of resolved Balmer emission, the spectrum has evolved to show broad absorption features at about 440, 475, 510, and 610 nm (rest-frame for z = 0.062), while the Balmer emission lines remain. The spectrum obtained on Nov. 12.3 UT is remarkably similar to the spectrum of the peculiar type-Ia supernova 2002ic (cf. IAUC 8019, 8151, 8157, 8161) obtained on 2002 Dec. 27 (Hamuy et al. 2003, Nature 424, 651). It is concluded that SN 2005gj is a 1991T-like type-Ia event interacting with circumstellar material and giving rise to the resolved emission lines on top of the characteristic type-Ia spectrum. This is the second clear detection of hydrogen in the spectrum of a type-Ia supernova. The preliminary SDSS-II light curve indicates that it has just passed maximum light (at magnitude r about 17.2). The study of SN 2005gj at all wavelengths is urged, as such data could help to understand better the nature of the progenitors of thermonuclear supernovae.