RECONCILING THE INFRARED CATASTROPHE AND OBSERVATIONS OF SN 2011fe

The observational effects of the "Infrared Catastrophe" are discussed in view of the very late observations of the Type Ia SN 2011fe. Our model spectra at 1000 days take non-local radiative transfer into account and find that this has a crucial impact on the spectral formation. Although rapid cooling of the ejecta to a few 100 K occurs also in these models, the late-time optical/NIR flux is brighter by 1-2 mag due to redistribution of UV emissivity, resulting from non-thermal excitation and ionization. This effect brings models into better agreement with late-time observations of SN 2011fe, and other SNe Ia, and offers a solution to the long-standing discrepancy between models and observations. The models show that spectral formation shifts from Fe II and Fe III at 300 days to Fe I at 1000 days, which explains the apparent wavelength shifts seen in SN 2011fe. We discuss the effects of time dependence and energy input from Co-57, finding both to be important at 1000 days.