  A growing subset of Type Ia supernovae (SNe Ia) show evidence for unexpected
interaction with a dense circumstellar medium (SNe Ia-CSM). The precise nature
of the progenitor, however, remains debated owing to spectral ambiguities
arising from a strong contribution from the CSM interaction. Late-time spectra
offer potential insight if the post-shock cold, dense shell becomes
sufficiently thin and/or the ejecta begin to cross the reverse shock. To date,
few high-quality spectra of this kind exist. Here we report on the late-time
optical and infrared spectra of the SNe~Ia-CSM 2012ca and 2013dn. These SNe
Ia-CSM spectra exhibit low [Fe III]/[Fe II] ratios and strong [Ca II] at late
epochs. Such characteristics are reminiscent of the super-Chandrasekhar-mass
(SC) candidate SN 2009dc, for which these features suggested a low-ionisation
state due to high densities, although the broad Fe features admittedly show
similarities to the blue "quasi-continuum" observed in some core-collapse SNe
Ibn and IIn. Neither SN 2012ca nor any of the other SNe Ia-CSM show evidence
for broad oxygen, carbon, or magnesium in their spectra. Similar to the
interacting Type IIn SN 2005ip, a number of high-ionisation lines are
identified in SN 2012ca, including [S III], [Ar III], [Ar X], [Fe VIII], [Fe
X], and possibly [Fe XI]. The total bolometric energy output does not exceed
10^51 erg, but does require a large kinetic-to-radiative conversion efficiency.
All of these observations taken together suggest that SNe Ia-CSM are more
consistent with a thermonuclear explosion than a core-collapse event, although
detailed radiative transfer models are certainly necessary to confirm these
results.
