Rings and Halos in the Mid-Infrared: The Planetary Nebulae NGC 7354 and
  NGC 3242
  We present images of the planetary nebulae (PNe) NGC 7354 and NGC 3242 in
four mid-infrared (MIR) photometric bands centred at 3.6, 4.5, 5.8 and 8.0
microns; the results of observations undertaken using the Spitzer Space
Telescope (SST). The resulting images show the presence of a halo and rings in
NGC 3242, as previously observed through narrow band imaging at visual
wavelengths, as well as evidence for a comparable halo and ring system in NGC
7354. This is the first time that a halo and rings have been observed in the
latter source.
  We have analysed the formation of halos as a result of radiatively
accelerated mass loss in the AGB progenitors. Although the models assume that
dust formation occurs in C-rich environments, we note that qualitatively
similar results would be expected for O-rich progenitors as well. The model
fall-offs in halo density are found to result in gradients in halo surface
brightness which are similar to those observed in the visible and MIR.
