Abstract
The results of a study of the chemical compositions of Galactic planetary nebulae taking into account two types of inhomogeneity in the nebular gas density in their envelopes are reported. New analytical expressions for the ionization correction factors have been derived and are used to determine the chemical compositions of the nebular gas in Galactic planetary nebulae. The abundances of He, N, O, Ne, S, and Ar have been found for 193 objects. The Y–Z diagrams for various He abundances are analyzed for type II planetary nebulae separately and jointly with HII regions. The primordial helium abundance Y p and enrichment ratio dY/dZ are determined, and the resulting values are compared with the data of other authors. Radial abundance gradients in the Galactic disk are studied using type II planetary nebulae.
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