THE [NE-III]-[O-II] SPECTRUM AS AN IONIZATION INDICATOR IN NEBULAE

We examine the usefulness of the [Ne III]-[O II] spectrum as a helium ionization correction factor (ICF) indicator in low-density nebulae such as H II regions. We present the results of an extensive grid of photoionization simulations which have a "blister"-like geometry. Fully filled constant pressure gas, Orion dust, and H II region abundances are used, along with a very wide variety of ionizing continua. We find that, as suggested by the ionization potentials, the He ICF is small when the [Ne III] lambda-3869/[O II] lambda-3727 ratio, an easily measured line pair, reaches greater-than-or-equal-to 0.20. We also examine the utility of the "eta" parameter, a ratio of ratios of oxygen and sulfur lines. We show that this ratio is probably strongly affected by low-temperature dielectronic recombination, a basic process which has no reliable rate coefficients for third-row elements. Although it is likely that the eta parameter will someday be the more powerful ICF indicator (because it has no direct dependence on abundance) it is not now possible to reliably calibrate eta by reference to photoionization calculations. Although the [Ne III]/[O II] ratio proposed here does have the disadvantage of being abundance sensitive, all relevant atomic data are in place and a comparison with model calculations is possible.