The NE III-O II forbidden-line spectrum as an ionization indicator in nebulae

The usefulness of the Ne III-O II forbidden-line spectrum as a helium ionization correction factor (ICF) indicator in low-density nebulae, such as H II regions, is examined, and the results of an extensive grid of photoionization simulations which have a 'blister'-like geometry are presented. Fully filled constant pressure gas, Orion dust, and H II region abundances are used, along with a very wide variety of ionizing continua. As suggested by the ionization potentials, the He ICF is small when the Ne III 3869/O II 3727 forbidden line ratio, an easily measured line pair, reaches above 0.20. The 'eta' parameter, a ratio of ratios of oxygen and sulfur lines, is probably strongly affected by low-temperature dielectronic recombination, a basic process which has no reliable rate coefficients for third-row elements.