Reconciling the infrared catastrophe and observations of SN 2011fe

TL;DR

This paper models late-time spectra of SN 2011fe, showing that non-local radiative transfer and non-thermal processes explain the observed optical/NIR brightness and spectral shifts, resolving the infrared catastrophe discrepancy.

Contribution

It introduces detailed non-local radiative transfer models that account for non-thermal effects, reconciling the infrared catastrophe with observations of SN 2011fe.

Findings

Late-time optical/NIR flux is brighter by 1-2 magnitudes due to UV redistribution.

Spectral formation shifts from Fe II/Fe III to Fe I at 1000d.

Non-thermal excitation and energy input from 57Co are crucial at late times.

Abstract

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 1000d 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 magnitudes 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 Type Ia supernovae, 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 300d to Fe I at 1000d, which explains the apparent wavelength shifts seen in SN2011fe. We discuss effects of time dependence and energy input from 57Co, finding both to be important at 1000d.