SIMBAD references

We present 3.6-, 4.5-, 5.8- and 8.0-µm photometric mapping of 18 galactic planetary nebulae, based on observations taken with the Spitzer Space Telescope. These are shown to have morphologies which are sometimes quite different from those observed in the visible, with much of the emission arising outside the ionized shells. There is also evidence for a change in nebular sizes between the differing photometric bands. An analysis of mid-infrared (MIR) colours suggests that many nebulae have dust/polycyclic aromatic hydrocarbon (PAH) emission components, and it seems likely that longer wave MIR fluxes (in particular) are associated with PAH emission features. Such features are likely to be associated, in addition, with photodissociative regimes, where shock fragmentation of dust may lead to the replenishment of smaller PAH grains.

Finally, we note that the source Ap 2-1 possesses a particularly interesting structure, and may represent a case in which the nebular shell is ploughing through an enveloping HII region. Similarly, it appears that central emission in M 2-48 may arise from a warm and dusty collimating disc.