Imaging spectrophotometry of planetary nebulae.

摘要

Flux-calibrated, two-dimensional emission-line and diagnostic line-ratio maps have been constructed for three planetary nebulae (PNs): NGC 6720, NGC 7662, and NGC 7009. Imaging Fabry-Perot interferometry, narrowband filter imaging, and longslit spectrophotometry are combined into a hybrid data set that takes advantage of the strengths of each technique. The final data set has good spatial coverage (5{dollar}spprimetimes5spprime{dollar}) and angular resolution (2{dollar}sp{lcub}primeprime{rcub}{dollar}), good spectral resolution on closely spaced emission lines (4A) and accurate spectrophotometry ({dollar}pm{dollar}5%). These observations yielded two-dimensional maps of line emissivity, diagnostic line ratios (e.g. (O III) {dollar}lambda{dollar}5007/H{dollar}beta{dollar}, (N II) {dollar}lambda{dollar}6584/H{dollar}alpha{dollar}), (N II) electron temperature, (S II) electron density and Balmer decrement across the nebulae. The scientific objective was to study the structure and evolution of PNs.; The initial target was NGC 6720 (the Ring Nebula). There are three principal results. (1) The Ring Nebula is ionization-bounded and there is a clumpy ionization front around the entire outer edge of the nebula. (2) The prominent, (O I) -bright filaments were expected to be cool and dense, but they are not--no differences were seen between the temperature and density in the filaments and the surrounding nebula. (3) The density enhancements along the minor axis suggest the "ring" of the nebula is intrinsically oval-shaped, rather than being a tilted circular ring.; In NGC 7662 and NGC 7009, the low-ionization microstructures do follow the predictions: the (O I) -bright knots are dense structures (except for the FLIERS, as expected). The morphology, density structure (i.e. the dense rim), and ionization fronts in NGC 7662 and NGC 7009 all fit the predictions of radiative-hydrodynamical models. However, the central star observations of NGC 7662 and NGC 7009 indicate that these PNs are significantly older than the models they match most closely. It turns out that the central stars of these two PNs have lower masses than the central stars assumed by the models. This contradiction is not easily resolved, and it may be premature to say that the models reproduce real PNs.