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.
