Spitzer Mid-Infrared Observations of Seven Bipolar Planetary Nebulae
  We have investigated the mid-infrared (MIR) and visual structures of seven
bipolar planetary nebulae (BPNe), using imaging and spectroscopy acquired using
the Spitzer Space Telescope (SST), and the Observatorio Astronomico Nacional in
Mexico. The results show that the sources are more extended towards longer MIR
wavelengths, as well as having higher levels of surface brightness in the 5.8
and 8.0 microns bands. It is also noted that the 5.8/4.5 and 8.0/4.5 microns
flux ratios increase with increasing distance from the nuclei of the sources.
All of these latter trends may be attributable to emission by polycyclic
aromatic hydrocarbons (PAHs) and/or warm dust continua within circum-nebular
photo-dissociation regions (PDRs). A corresponding decrease in the flux ratios
8.0/5.8 microns may, by contrast, arise due to changes in the properties of the
PAH emitting grains. We note evidence for possible 8.0 microns ring-like
structures in the envelope of NGC 2346, located in a region beyond the minor
axis limits of the ionized envelope. An analysis of the inner two rings shows
that whilst they have higher surface brightnesses at longer MIR wavelengths,
they are relatively stronger (compared to underlying emission) at 3.6 and 4.5
microns. There is also evidence for point reflection symmetry along the major
axis of the outflow.
