Ionization correction factors for low-excitation gaseous nebulae

In a gaseous nebula, the determination of the abundance of an element X, relative to hydrogen, involves two separate steps. One is the measurement of the abundances of each stage of ionization of X which has an observable emission line, while the second step is concerned with the relation of ionic abundances to elemental abundances. After the determination of the observable ionic abundances, it is necessary to estimate 'ionization correction factors' (ICFs), which correct for the ionic stages that have no observable lines. For some elements, the unobservable stages are the most important for the element. The ICFs are estimated from theoretical models or by empirical rules. The present study is concerned with ICFs as determined by models whose parameters are varied over a wide range of parameter space. Attention is given to parameters of a model, results for various ions, the determination of neutral H within the nebula, and nebular and stellar temperatures.