While the theory of the radiation of planetary nebulae (PN) appears satisfactory, the quality of observations often leaves much to be desired. Even in close doublets, relative line intensities may be seriously in error, probably owing to erroneous estimates of the continuum. These errors affect the determinations of electron density.
The radio and optical images of PN are much alike. More surprises may come from emission in the 21-cm and molecular lines. Infrared observations suggest that dust is well mixed with the gas; in the younger planetaries the dust may be hotter and the infrared emission is stronger.
Determinations of the physical parameters of PN are hampered by incomplete and unreliable spectrophotometric data, and by structural peculiarities. Computer models have so far been too primitive. Both density and ionization must vary with radius, and are interdependent; hence, shell models are required. Studies of small-scale condensations have centred on the comet-like structures in NGC 7293. The physical conditions there are probably close to those in the main body of this nebula, but spectra must resolve this matter.
The evolution of PN shells is observed over 4 or 5 orders of magnitude in density. However, densities above 108 cm−3 are not observed and the evolution of PN progenitors (out of old red giants?) remains speculative. Age estimates based on a constant rate of expansion are clearly too primitive. The increase of the ratio of the Hell λ 4686 to Hβ with time does not necessarily imply an evolutionary rise in temperature of the PN nucleus.
KeywordsInfrared Emission Planetary Nebula Structural Peculiarity Spectrophotometric Data Relative Line Intensity
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