  SN 2009dc shares similarities with normal Type Ia supernovae, but is clearly
overluminous, with a (pseudo-bolometric) peak luminosity of log(L) = 43.47
[erg/s]. Its light curves decline slowly over half a year after maximum light,
and the early-time near-IR light curves show secondary maxima, although the
minima between the first and second peaks are not very pronounced. Bluer bands
exhibit an enhanced fading after ~200 d, which might be caused by dust
formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are
dominated by intermediate-mass elements and unburned material at early times,
and by iron-group elements at late phases. Strong C II lines are present until
~2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta
velocities are significantly lower than in normal and even subluminous SNe Ia.
No signatures of CSM interaction are found in the spectra. Assuming that the
light curves are powered by radioactive decay, analytic modelling suggests that
SN 2009dc produced ~1.8 solar masses of 56Ni assuming the smallest possible
rise time of 22 d. Together with a derived total ejecta mass of ~2.8 solar
masses, this confirms that SN 2009dc is a member of the class of possible
super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A
study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a
tendency of these SNe to explode in low-mass galaxies. A low metallicity of the
progenitor may therefore be an important pre-requisite for producing
superluminous SNe Ia. We discuss a number of explosion scenarios, ranging from
super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf
mergers and Type I 1/2 SNe to a core-collapse origin of the explosion. None of
the models seem capable of explaining all properties of SN 2009dc, so that the
true nature of this SN and its peers remains nebulous.
