Chemical Abundance Determinations in Gaseous Nebulae

Abstract

The determination of the chemical abundances in planetary nebulae and HII regions from the observed intensities of emission lines usually needs a correction for the unseen ionization stages. An empirical correction was first introduced by Peimbert and Costero (1969) as a factor ICF (ionization correction factor) which was obtained assuming that ions with similar ionization potentials are equally populated. Recently, Natta et al. (1980) discussed the problem of sulphur abundance in nebulae and proposed an ICF for sulphur which takes into account the radiation field as well as the ionization equilibrium of 0+, 0++, S++ and S+3. However they did not consider the effect of the charge exchange reactions which can be very important in determining the ionization equilibrium of the gas (Pequignot et al., 1978).