Fourteen months of observations of the possible super-Chandrasekhar mass Type Ia Supernova 2009dc

In this paper, we present and analyse optical photometry and spectra of the\nextremely luminous and slowly evolving Type Ia supernova (SN Ia) 2009dc, and\noffer evidence that it is a super-Chandrasekhar mass (SC) SN Ia and thus had a\nSC white dwarf (WD) progenitor. Optical spectra of SN 2007if, a similar object,\nare also shown. SN 2009dc had one of the most slowly evolving light curves ever\nobserved for a SN Ia, with a rise time of ~23 days and Delta m_15(B) = 0.72\nmag. We calculate a lower limit to the peak bolometric luminosity of ~2.4e43\nerg/s, though the actual value is likely almost 40% larger. Optical spectra of\nSN 2009dc and SN 2007if obtained near maximum brightness exhibit strong C II\nfeatures (indicative of a significant amount of unburned material), and the\npost-maximum spectra are dominated by iron-group elements. All of our spectra\nof SN 2009dc and SN 2007if also show low expansion velocities. However, we see\nno strong evidence in SN 2009dc for a velocity "plateau" near maximum light\nlike the one seen in SN 2007if (Scalzo et al. 2010). The high luminosity and\nlow expansion velocities of SN 2009dc lead us to derive a possible WD\nprogenitor mass of more than 2 M_Sun and a Ni-56 mass of about 1.4-1.7 M_Sun.\nWe propose that the host galaxy of SN 2009dc underwent a gravitational\ninteraction with a neighboring galaxy in the relatively recent past. This may\nhave led to a sudden burst of star formation which could have produced the SC\nWD progenitor of SN 2009dc and likely turned the neighboring galaxy into a\n"post-starburst galaxy." No published model seems to match the extreme values\nobserved in SN 2009dc, but simulations do show that such massive progenitors\ncan exist (likely as a result of the merger of two WDs) and can possibly\nexplode as SC SNe Ia.\n