EARLY PHASE OBSERVATIONS OF EXTREMELY LUMINOUS TYPE Ia SUPERNOVA 2009dc

We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is Δm ₁₅(B) = 0.65 ± 0.03, which is one of the slowest among SNe Ia. The peak V-band absolute magnitude is estimated to be MV = -19.90 ± 0.15 mag if no host extinction is assumed. It reaches MV = -20.19 ± 0.19 mag if we assume the host extinction of AV = 0.29 mag. SN 2009dc belongs to the most luminous class of SNe Ia, like SNe 2003fg and 2006gz. Our JHKs -band photometry shows that this SN is also one of the most luminous SNe Ia in near-infrared wavelengths. We estimate the ejected ⁵⁶Ni mass of 1.2 ± 0.3 M sun for the no host extinction case (and of 1.6 ± 0.4 M sun for the host extinction of AV = 0.29 mag). The C II λ6580 absorption line remains visible until a week after the maximum brightness, in contrast to its early disappearance in SN 2006gz. The line velocity of Si II λ6355 is about 8000 km s⁻¹ around the maximum, being considerably slower than that of SN 2006gz. The velocity of the C II line is similar to or slightly less than that of the Si II line around the maximum. The presence of the carbon line suggests that the thick unburned C+O layer remains after the explosion. Spectropolarimetric observations by Tanaka et al. indicate that the explosion is nearly spherical. These observational facts suggest that SN 2009dc is a super-Chandrasekhar mass SN Ia.