Chemical Compositions of Selected Planetary Nebulae.

By methods similar to those used in a previous study of NGC 7027, analyses are carried out for NGC 1535, 2022, 2165, 6741, IC 5217, NGC , 2392, and 7662 and Anon +21li3100, +390 The observations employed are primarily those made by Minkowski and the writer at the Mount Wilson Observatory. The effects of space absorption and of filamentary structure are discussed, but the most serious errors appear to be those arising from trying to allow for the distribution of atoms among the various stages of ionization. These difficulties are particularly troublesome for nitrogen, which is observed only in the lowest stages of ionization. The neon/oxygen ratio appears to be smaller than in the stars, whereas the sulfur/oxygen ratio appears to be larger. The hydrogen/helium ratio appears to be about 5. No conclusive evidence is found for actual composition variations from one object to another, although the hydrogen/helium and particularly the nitrogen/oxygen ratios present some puzzling questions. If abundance differences are ruled out, we must postulate in certain objects, such as Anon +21h3100, +390 and NGC 2440, the simultaneous presence of high- and low-excitation filaments.