The Halo and Rings of the Planetary Nebula NGC 40 in the Mid-Infrared
  We present imaging and spectroscopy of NGC 40 acquired using the Spitzer
Space Telescope (Spitzer), and the Infrared Space observatory (ISO). 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 H2 transitions. It is suggested
that an apparent jet-like structure to the NE of the halo represents one of the
many emission spokes that permeate the shell. 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 microns and 30 microns 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
