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We have acquired narrowband imaging of NGC 3587 (the Owl Nebula) in the transitions [O II] λλ3727, 3729, H I λ4861, [O III] λ5007, H I λ6563, [N II] λ6584, [S II] λ6717, and [S II] λ6731. As a result, we are able to evaluate the variation of excitation and electron density over the projected envelope of the source. We propose that the Owl Nebula consists of four primary shells: an internal, tilted, barrel-like component responsible for higher excitation emission; two much more uniform, spherically symmetric structures, CSCI and CSCII. These, finally, are enveloped by a much lower intensity, lower excitation halo, dubbed CSCIII. A large proportion of the low-excitation emission appears to be associated with the periphery of CSCI, and it is conceivable that this is, physically speaking, a relatively thin-shelled structure. [S II] density mapping appears to indicate that ne is preferentially enhanced toward the northern periphery of the shell, in a regime where low-excitation line strengths are also preferentially enhanced. We suggest that such trends may arise through northerly shocking of the shell CSCI. There is, in addition, some evidence for an east-west dichotomy in density structure, which may reflect variable sampling of the higher density, barrel-like component. Mean densities are low, and of the order ne = 590 cm-3.

We have acquired narrowband imaging of NGC 2346 in the transitions H I λ6563 and λ4861 and [O III] λ5007. As a result, we are able to evaluate the variation of both excitation and extinction over the projected central parts of the nebular shell. Extinction appears to be surprisingly uniform, and there is (in particular) little evidence for the reddening asymmetries proposed in previous analyses. Cusplike enhancements in Av at the periphery of the source are attributed to a layer of extincting material at the limits of the H II region, implying a value of major-axis reddening ΔAv < 0.5 mag. Similar extinctions are also deduced from an analysis of infrared and millimeter-wavelength observations. It is unclear what proportion of this reddening may be attributed to the molecular belt, although it seems likely that this feature contributes ~0.3 mag of extinction in the northern lobe and explains the north-south asymmetries noted in optical images. Given that levels of local extinction are quite modest, it seems likely that the larger part of the observed reddening (Av ≈ 2 mag) arises from intervening interstellar material, a conclusion that is again at variance with previous analyses. Such a presumption would explain the relative uniformity in extinction over the face of the nebula, and similarity between distances estimated from extinction and those determined through alternative analyses. If this is accepted, however, then it follows that prior estimates of central star extinction must be greatly in error. One possible origin for this error is noted from Hubble Space Telescope imagery in [N II] λ6584, where it is clear that a secondary star is located close to the presumed A-type central star, a component that was not allowed for in previous analyses. It is unclear whether this star is physically associated with the core binary system.

We have acquired imaging of the bipolar outflow M2-9 in the transitions [O III] λ5007, H I λ6563, [N II] λ6584, and [S II] λλ6717, 6731. As a result, we are able to map both excitation and density over the face of the nebula. We find that underlying densities in the lobes increase to ne ~ 1.6 × 103 cm-3 at distances rp ~ 12''–15'' from the central star, after which they again decrease to ne 27''. Superposed upon this are various density knots, associated with the visual condensations, in which peak values of ne are of order 6.3 × 103 cm-3. These appear to be responsible for low-excitation shadowing of the outer envelope. All of these features (the density peaks, and broad underlying density variation) are shown to be consistent with a particular model of shock excitation, in which an interior wind is shock-interacting with an external neutral envelope. Such a model appears also to be consistent with trends in [O III]/Hα and [S II]/Hα, which (for the northern lobe at least) appear most consistent with shock excitation.

The planetary nebula NGC 2392 possesses an extremely unusual spectrokinematic morphology, characterized by several strange and elusive structures. In this paper, extensive narrowband imaging of the source is used to investigate the shell excitation structure, while [O III] and [S II] images are ratioed to provide fully sampled mapping of electron density and temperature. As a result, we are able to confirm the presence of low densities in the various filamentary structures, an appreciable radial decrease in density in the outer shell, and finally, a probable regime of higher shell densities at the interface of the bright inner shell and lower intensity outer halo. Although temperatures are more uniform, we also find evidence for a significant temperature enhancement close to the inner-outer shell interface, and it is proposed that both anomalies (in density and temperature) arise in a postshock cooling regime. Finally, present imaging and prior spectroscopy are used to construct a revised model of the source, in which it is argued that the outer filamentary structure occupies a disklike regime, congruent with the principal plane of the outer spheroid. Individual filaments arise from low-excitation density contrasts, created through the interaction between the outer shell and a precessing jet.

We have acquired [O III] λ5007, H I λλ6563 and 4861, and [S II] λλ6717 and 6731 imaging of the bipolar outflow source NGC 6445. As a result, we find that density is preferentially enhanced within an interior optical/radio emission annulus and takes a maximum value ~1.1 × 103 cm-3. Visual extinction appears also to be enhanced within this regime, taking values ~0.8 mag greater than for the enveloping shell, a result that is shown to imply ratios Av/N(H2) comparable to that of NGC 7027 and at least 4 times greater than for the interstellar medium. Finally, we investigate the variation of excitation through the projected nebular envelope. While the nucleus possesses excitation properties consistent with normal (radiatively excited) sources, it appears that line ratios toward the source periphery may indicate the presence of appreciable shock activity.