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The Structure of the Planetary Nebula NGC 2371 in the Visible and Mid-Infrared

We investigate the structure of the planetary nebula (PN) NGC 2371 using [OIII]-5007 imaging taken with the Jacobus Kapteyn 1.0 m telescope, and [NII]-6584, [OIII]-5007 and Ha results acquired with the Hubble Space Telescope (HST). These are supplemented with archival mid-infrared (MIR) observations taken with the Spitzer Space Telescope (Spitzer). We note the presence of off-axis low-ionization spokes along a PA of 65 degrees, and associated collars of enhanced [OIII] emission. The spokes appear to consist of dense condensations having low-excitation tails, possibly arising due to UV shadowing and/or ram-pressure stripping of material. Line ratios imply that most of the emission arises through photo-ionisation, and is unlikely to derive from post-shock cooling regions. An analysis of these features in the MIR suggests that they may also be associated with high levels of emission from polycyclic aromatic hydrocarbons (PAHs), together with various permitted and forbidden line transitions. Such high levels of PAH emission, where they are confirmed, may develop as a result of preferentially enhanced FUV pumping of the molecules, or shattering of larger grains within local shocks. Although H2 emission may also contribute to these trends, it is argued that shock-excited transitions would lead to markedly differing results. We finally note that thin filaments and ridges of [OIII] emission may indicate the presence of shock activity at the limits of the interior envelope, as well as at various positions within the shell itself. We also note that radially increasing fluxes at 3.6, 5.8 and 8.0 microns, relative to the emission at 4.5 microns, may arise due to enhanced PAH emission in external photo-dissociative regions (PDRs).