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Ionization correction factors (ICFs) are ratios that relate observable ionization measurements of element to their total abundances. These can be used for observations of interstellar medium (ISM) regions such as HII regions to determine the relative abundances of constituents. Often the observable spectral lines are from ions, so the strategy is to estimate total abundances from these available observations.
A typical ICF is a ratio between two ratios: one ratio being the desired ratio between the total abundances of the element and hydrogen and the other ratio being the abundances of the element's and hydrogen's (observable) ions. The aim is to find out the element's total abundance relative to that of hydrogen, the relative abundances of their ions being determined from observable lines. Functions have been devised to calculate ICFs for various elements in various specific situations, the functions applied to other observables (including spectral lines of other elements). The appropriate function may depend upon characteristics of the cloud/object under analysis, including its type and its temperature regime. These functions to generate ICFs are developed from a combination of theory, modeling of photoionization, and data form observations where some of the target abundances can be determined by other means.