Excitation and Density Mapping of NGC 3587

Abstract

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.