On the nature of Type IIn/Ia–CSM supernovae: optical and near-infrared spectra of SN 2012ca and SN 2013dn

A growing subset of Type Ia supernovae (SNe Ia) show evidence for unexpected\ninteraction with a dense circumstellar medium (SNe Ia-CSM). The precise nature\nof the progenitor, however, remains debated owing to spectral ambiguities\narising from a strong contribution from the CSM interaction. Late-time spectra\noffer potential insight if the post-shock cold, dense shell becomes\nsufficiently thin and/or the ejecta begin to cross the reverse shock. To date,\nfew high-quality spectra of this kind exist. Here we report on the late-time\noptical and infrared spectra of the SNe~Ia-CSM 2012ca and 2013dn. These SNe\nIa-CSM spectra exhibit low [Fe III]/[Fe II] ratios and strong [Ca II] at late\nepochs. Such characteristics are reminiscent of the super-Chandrasekhar-mass\n(SC) candidate SN 2009dc, for which these features suggested a low-ionisation\nstate due to high densities, although the broad Fe features admittedly show\nsimilarities to the blue "quasi-continuum" observed in some core-collapse SNe\nIbn and IIn. Neither SN 2012ca nor any of the other SNe Ia-CSM show evidence\nfor broad oxygen, carbon, or magnesium in their spectra. Similar to the\ninteracting Type IIn SN 2005ip, a number of high-ionisation lines are\nidentified in SN 2012ca, including [S III], [Ar III], [Ar X], [Fe VIII], [Fe\nX], and possibly [Fe XI]. The total bolometric energy output does not exceed\n10^51 erg, but does require a large kinetic-to-radiative conversion efficiency.\nAll of these observations taken together suggest that SNe Ia-CSM are more\nconsistent with a thermonuclear explosion than a core-collapse event, although\ndetailed radiative transfer models are certainly necessary to confirm these\nresults.\n