NEBULAR ABUNDANCE ERRORS

The errors inherent to the use of the standard ''ionization correction factor'' (''i(CF)'') method of calculating nebular conditions and rel ative abundances of H, He, N, O, Ne, S, and Ar in emission line nebula e have been investigated under conditions typical for planetary nebula e. The photoionization code CLOUDY was used to construct a series of m odel nebulae with properties spanning the range typical of PNe. Its ra dial ''profiles'' of bright, frequently observed optical emission line s were then summed over a variety of ''apertures'' to generate sets of emission Line measurements. These resulting line ratios were processe d using the i(CF) method to ''derive'' nebular conditions and abundanc es. We find that for lines which are summed over the entire nebula the i(CF)-derived abundances differ from the input abundances by less tha n or equal to 25% for He and O up to less than or equal to 25% or more for Ne, S, and Ar. For resolved observations, however, the discrepanc ies are often much larger and are systematically variable with radius. This effect is especially pronounced in low-ionization zones where ni trogen and oxygen are neutral or once-ionized such as in FLIERs, ansae and ionization fronts. We argue that the reports of stellar-enriched N in the FLIERs of several PNe are probably specious. (C) 1997 America n Astronomical Society.