2011MNRAS.411.1245R

We present the imaging and spectroscopy of NGC 40 acquired using the Spitzer Space Telescope and the Infrared Space Observatory. These are used to investigate the nature of emission from the central nebular shell, from the nebular halo and from the associated circumnebular rings. It is pointed out that a variety of mechanisms may contribute to the mid-infrared (MIR) fluxes, and there is evidence for a cool dust continuum, strong ionic transitions and appreciable emission by polycyclic aromatic hydrocarbons (PAHs). Prior observations at shorter wavelengths also indicate the presence of warmer grains and the possible contribution of H₂transitions. Two of these components (warm dust with T_GR∼ 500-620 K and PAH emission bands) appear capable, in combination, of explaining the infrared colours of the rings and halo, although the flux ratios are also consistent with shock-excited H₂v= 0-0 emission. It is noted that whilst the surface brightness of the rings is greater in the longer wave (5.8- and 8.0-µm) photometric channels, their fractional fluxes (when compared to the halo) are greater at 3.6 and 4.5 µm, a trend which is similar to those observed in other planetary nebulae. It is also apparent that the relative intensities of the rings are greater than is observed for the Hα+[N II] transitions. It is suggested that an apparent jet-like structure to the north-east of the halo represents one of the many emission spokes that permeate the shell and which are observed for the first time in these MIR results. The spokes are likely to be caused by the percolation of UV photons through a clumpy interior shell, whilst the jet-like feature is enhanced due to locally elevated electron densities, a result of interaction between NGC 40 and the interstellar medium. It is finally noted that the presence of the PAH, 21- and 30-µm spectral features testifies to appreciable C/O ratios within the main nebular shell. Such a result is consistent with abundance determinations using collisionally excited lines, but not with those determined using optical recombination lines.