2010MNRAS.405.2179P

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-µm bands. It is also noted that the 5.8µm/4.5µm and 8.0µm/4.5µm 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 circumnebular photodissociation regions (PDRs). A corresponding decrease in the flux ratios 8.0µm/5.8µm may, by contrast, arise due to changes in the properties of the PAH emitting grains. We note evidence for 8.0µm 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 4.5µm. There is also evidence for point-reflection symmetry along the major axis of the outflow. Finally, the fall-off in surface brightness along the nebular minor axis suggests that progenitor mass-loss rates were more-or-less constant. NGC 6905 shows evidence for a centrally located and unresolved MIR emission excess. We show that whilst flux ratios are inconsistent with stellar and ionized gas components of emission, the emission can be explained in terms of shock-excited H₂, or grain continuum and PAH emission bands. We provide the deepest image so far published of the outer visual structure of NGC 6445, and compare these results with Spitzer imaging at 8.0µm. The distributions are quite distinct, and may indicate collimation of the outer ionized emission by a cylindrical PDR. We finally note that the MIR colours of the lobes and nuclei of the sources are radically different. It is also shown that 3.6-µm emission in BPNe differs from that for the generality of planetary nebulae. We discuss various mechanisms which may contribute to these differences of colour.