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MIPSGAL 24 μm Observations of Galactic Planetary Nebulae

We have obtained 24 μm 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 μm 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 μm 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 24μm 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 μm 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.