Chemical compositions of planetary nebulae

The chemical compositions of 41 planetary nebulae are established on the basis of an analysis of image tube scanner measurements, supplemented by IUE data. An attempt is made to calculate a theoretical model that gives satisfactory values of the observed intensities, as well as a number of excitation-sensitive ratios, in order to obtain ionization correction factors applicable to the ionic concentrations that will lead to total abundance values. The quality of individual nebular analyses varies, depending on the completeness and accuracy of the observational data and the complexity of the nebular structure. Quality estimates are assigned for each element in each nebula, and an overall reliability index is given to each object. It is concluded that the confidence with which one can speak of the chemical composition of a nebula varies significantly.