MIPSGAL 24 {\mu}m Observations of Galactic Planetary Nebulae
  We have obtained 24 {\mu}m imaging, profiles and fluxes for 224 planetary
nebulae (PNe) lying within the limits of the Spitzer MIPSGAL survey. It is
noted that most of the PNe having extended 24 {\mu}m emission also possess
circular morphologies, suggesting that the emission derives from cool grains
located within the AGB mass-loss regimes. Certain of these halos are found to
have a surface brightness fall-off which may be consistent with secularly
invariant mass-loss within the PNe progenitors. By contrast, the 8.0 {\mu}m
envelopes are detected out to smaller distances from the nuclei, and have a
steeper rate of surface brightness fall-off; a phenomenon which may arise from
changes in the excitation of polycyclic aromatic hydrocarbons (PAHs) within
external photo-dissociation regimes (PDRs). Our 24{\mu}m fluxes are compared to
those in previously published studies, and this appears to indicate that many
of the prior fluxes have been underestimated; a disparity may imply that
previous aperture sizes were too small. We have also combined our 24 {\mu}m
fluxes with measures at shorter mid-infrared (MIR) wavelengths, taken with the
Infrared Array Camera (IRAC). These are used to investigate the positioning of
PNe within the IRAC-MIPSGAL colour planes. The [8.0]-[24] and [5.8]-[24]
colours are found to be large, and extend over the respective ranges 3.4-8.7
mag, and 5.4-10.3 mag; indices which are only explainable where a broad range
of mechanisms contribute to the fluxes, including PAH bands, cool dust
continua, and a variety of ionic transitions. These and other components also
affect the morphologies of the sources, and lead to wavelength dependent
changes in the widths of the profiles.
