Chemical compositions of planetary nebulae

Abstract

Image-tube scanner measurements, supplemented where possible by IUE data, are analyzed and reassessed as necessary to establish chemical compositions of 41 planetary nebulae. The objects cover a large range in surface brightness, structural complexity, and excitation. Two methods are employed. In one, theoretical models are calculated to represent individual line intensities and excitation-sensi-tive ratios such as [O n]/[0 m], He n/He I, and [Ne v]/[Ne in]. The " model abundances " thus derived correspond to the best fit of observed and calculated intensities within the framework imposed by the basic assumptions. For many objects fairly satisfactory representations are possible, but for a few nebulae, such as Hu 1-2 and NGC 2371-72, no satisfactory model could be obtained—at least none involving a simple geometry. Only the general level of excitation can be reproduced. In the second method we determine for each nebula the diagnostics, N e ,T e , and employ them to calculate the concentration, A(A r /)/Y(H +), for each observed ion. We try to calculate a theoretical model that gives satisfactory values of the observed intensities and the aforementioned excitation-sensitive ratios to obtain ionization correction factors which are then applied to the ionic concentrations to get total abundances. This procedure, applied, e.g., by Shields et al. to NGC 2440, is the one we favor. The mean logarithmic deviations between abundances derived by the two methods, A = (|ln Y el (ICF)/A el (model)|), is of the order of 0.10, except for N and S where A tends to be larger, primarily for reasons outlined in the text. The quality of the individual nebular analyses varies, depending on the completeness and accuracy of the observational data and the complexity of the nebular structure. We assign quality estimates for each element in each nebula and try to give an overall reliability index to each object. Thus, the confidence with which we can speak of the chemical composition of a nebula may vary over a fair range. The mean logarithmic abundances found for this sample of nebulae plus those individual objects previously reported are as follows: