Rings and haloes in the mid-infrared: the planetary nebulae NGC 7354 and NGC 3242

We present images of the planetary nebulae NGC 7354 and NGC 3242 in four mid-infrared (MIR) photometric bands centred at 3.6, 4.5, 5.8 and 8.0 μm, the results of observations undertaken using the Spitzer Space Telescope. The resulting images show the presence of a halo and rings in NGC 3242, as previously observed through narrow-band imaging at visual wavelengths, as well as evidence for a comparable halo and ring system in NGC 7354. This is the first time that a halo and rings have been observed in the latter source. Similarly, whilst partial rings may have been previously detected in the MIR (for the source NGC 3132), the present observations appear to constitute the first detections of complete ring systems outside of the visual wavelength regime. The halo/core emission ratios appear to be preferentially higher at MIR wavelengths than is the case in the visible, and show less steep fall-offs than those observed in the [O iii]λλ4959+ 5007 A transitions. The variation in surface brightness S with radial distance R, where this is approximated by the power-law relation S∝R−β, implies exponents 1.7 < β < 4 in the inner portions of the haloes, and 4.5 < β < 12 towards the outer limits of these structures. The value of β is also, in most cases, somewhat smaller at longer wavelengths. It is additionally noted that the 3.6 μm/4.5 μm, 5.8 μm/4.5 μm and 8.0 μm/4.5 μm flux ratios increase markedly away from the nuclei, and reach their maximum values in the haloes, where much of the longer wave flux may derive from dust band and continuum emission. An analysis of the rings suggests that some of them, at least, are likely to be associated with higher densities of dust particles, and that the gaseous and particle ring systems are likely to be spatially in register. Finally, we have analysed the formation of haloes as a result of radiatively accelerated mass loss in the AGB progenitors. Although the models assume that dust formation occurs in C-rich environments, we note that qualitatively similar results would be expected for O-rich progenitors as well. The model fall-offs in halo density are found to result in gradients in halo surface brightness which are similar to those observed in the visible and MIR.