Optical observations of type-IIP supernova 2004dj: Evidence for asymmetry of the 56Ni ejecta

Photometric and spectroscopic observations of the nearby type-IIP supernova 2004dj are presented. The 56 Ni mass in the envelope of SN 2004dj was estimated from the light curve to be ≈0.02 M ⊙ . This estimate is confirmed by modeling the H α luminosity. The H α emission line exhibits a strong asymmetry characterized by the presence of a blue component in the line with a shift of −1600 km s −1 at the early nebular phase. A similar asymmetry was found in the H β , [O I], and [Ca II] lines. The line asymmetry is interpreted as being the result of asymmetric 56 Ni ejecta. The H α profile and its evolution are reproduced in the model of an asymmetric bipolar 56 Ni structure for a spherical hydrogen distribution. The mass of the front 56 Ni jet is comparable to that of the central component and twice that of the rear 56 Ni jet. We point out that the asymmetric bipolar structure of 56 Ni ejecta is also present in SN 1999em, a normal type-IIP supernova.