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 Delta m(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 M-V = -19.90 +/- 0.15 mag if no host extinction is assumed. It reaches M-V = -20.19 +/- 0.19 mag if we assume the host extinction of A(V) = 0.29 mag. SN 2009dc belongs to the most luminous class of SNe Ia, like SNe 2003fg and 2006gz. Our JHK(s)-band photometry shows that this SN is also one of the most luminous SNe Ia in near-infrared wavelengths. We estimate the ejected Ni-56 mass of 1.2 +/- 0.3 M-circle dot for the no host extinction case (and of 1.6 +/- 0.4 M-circle dot for the host extinction of A(V) = 0.29 mag). The C II lambda 6580 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 lambda 6355 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.