Early phase observations of extremely luminous type Ia supernova 2009dc

Abstract

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 Δm15(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 0.15mag if no host extinction is assumed. It reaches MV = -20.19 0.19mag if we assume the host extinction of AV = 0.29mag. 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⊙ for the no host extinction case (and of 1.6 0.4 M⊙ for the host extinction of AV = 0.29mag). 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 8000kms-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 etal. indicate that the explosion is nearly spherical. These observational facts suggest that SN 2009dc is a super-Chandrasekhar mass SN Ia. © 2009 The American Astronomical Society.