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The Mid-Infrared and Optical Decay of SN 2011fe

We measure the decay rate of the mid-IR luminosity from type Ia supernova 2011fe between six months and one year after explosion using Spitzer/IRAC observations. The fading in the 3.6 micron channel is 1.48+/-0.02 mag/100d, which is similar to that seen in blue optical bands. The supernova brightness fades at 0.78+/-0.02 mag/100d in the 4.5 micron channel which is close to that observed in the near-IR. We argue that the difference is a result of doubly ionized iron-peak elements dominating the bluer IRAC band while singly ionized species are controlling the longer wavelength channel. To test this, we use Large Binocular Telescope spectra taken during the same phases to show that doubly ionized emission lines do fade more slowly than their singly ionized cousins. We also find that [Co III] emission fades at more than twice the radioactive decay rate due to the combination of decreasing excitation in the nebula, recombination and cobalt decaying to iron. The nebular emission velocities of [Fe III] and [Co III] lines show a smaller blue-shift than emission from singly ionized atoms. The Si II velocity gradient near maximum light combined with our nebular velocity measurements suggest SN 2011fe was a typical member of the `low velocity gradient' class of type Ia. Analyzing IRAC photometry from other supernovae we find that mid-IR color of type Ia events is correlated with the early light curve width and can be used as an indicator of the radioactive nickel yield.