On type IIn/Ia-CSM supernovae as exemplified by SN 2012ca
  We present the complete set of ultra-violet, optical and near-infrared
photometry and spectroscopy for SN 2012ca, covering the period from 6~days
prior to maximum light, until 531 days after maximum. The spectroscopic time
series for SN 2012ca is essentially unchanged over 1.5 years, and appear to be
dominated at all epochs by signatures of interaction with a dense circumstellar
medium rather than the underlying supernova (SN). At late phases, we see a near
infrared excess in flux which is possibly associated with dust formation,
although without any signs of accompanying line shifts. SN 2012ca is a member
of the set of type of the ambiguous IIn/Ia-CSM SNe, the nature of which have
been debated extensively in the literature. The two leading scenarios are
either a type Ia SN exploding within a dense CSM from a non-degenerate, evolved
companion, or a core-collapse SN from a massive star. While some members of the
population have been unequivocally associated with type Ia SNe, in other cases
the association is less certain. While it is possible that SN 2012ca does arise
from a thermonuclear SN, this would require a relatively high (between 20 and
70 per cent) efficiency in converting kinetic energy to optical luminosity, and
a massive (~2.3-2.6 Msun) circumstellar medium. On the basis of energetics, and
the results of simple modelling, we suggest that SN 2012ca is more likely
associated with a core-collapse SN. This would imply that the observed set of
similar SNe to SN 2012ca is in fact originated by two populations, and while
these are drawn from physically distinct channels, they can have
observationally similar properties.
