The structures of MASH and MASH II planetary nebulae at mid-infrared wavelengths

We present 3.6, 4.5, 5.8 and 8 μm mapping, images and profiles for 13 Macquarie/Anglo-Australian Observatory (AAO)/Strasbourg (MASH) and MASH II planetary nebulae (PNe). Most of these sources have mid-infrared (MIR) morphologies which are similar to those observed in the visible, although there are differences attributable to polycyclic aromatic hydrocarbon (PAH) emission features, warm dust continua and a variety of ionic and molecular transitions. Such transitions also lead to the development of more extended haloes at longer MIR wavelengths, likely associated with photo-dissociative regimes (PDRs) about the primary ionized envelopes. It is noted that the colour characteristics of the envelopes, and the relative lack of detection of the sources in the MIR, point to the nebulae being appreciably evolved, a result which is in accordance with a prior analysis of Hα results. Finally, it is suggested that differences between the MIR colours of the MASH sources on the one hand, and of less evolved PNe on the other, may arise because of an evolution in the numbers of PAH-carrying grains within the PDRs, and/or variations in the far-ultraviolet (FUV) energy density of the stellar radiation fields. The tendency for fluxes to increase with increasing MIR wavelength probably arises from variations in PAH emission strengths, as well as from the contribution of broader dust continua.