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) shows evidence via narrow emission lines for unexpected interaction with a dense circumstellar medium (SNe IIn/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, only a 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 candidate SN 2009dc, for which these features suggested a low-ionization 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 in this paper show evidence for broad oxygen, carbon, or magnesium in their spectra. Similar to the interacting Type IIn SN 2005ip, a number of high-ionization 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 1051 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.