IONIZATION CORRECTION FACTORS AND COMPOSITIONS OF H-II REGIONS

For several elements we present ionization correction factors (ICFs), the factors needed to convert the observed ionic abundances to elemental abundances. The ICFs are derived from nebular photoionization models using mainly plane-parallel stellar atmospheres, although others are considered briefly. We assume that observations of the ions O+, O+2, S+, and S+2 exist, so that log (O+/O+2) [= F(O)] and log(S+/S+2) [= F(S)] are known. The ICFs are given in the form of a power-series expansion in F(O) and F(S) with quadratic terms. The results for Ne and N agree well with the empirical ionization correction factors of Peimbert & Torres-Peimbert (1977), but typically differ by > dex (0.2) for S (for objects with high-temperature ionizing stars), Ar and Cl. The ICFs for Ar+2 and Cl+2 are very uncertain, and the elemental abundances are not known to better than a factor or two if the correction is an order of magnitude. The predicted ICFs are checked by comparison of elemental abundances among various lines of sight within the same object. Suitable observations exist for the Orion Nebula, 30 Doradus in the Large Magellanic Cloud, NGC 604 in M 33, and NGC 5471 in M 101. Using these ICFs, we find that the galactic and extragalactic H II regions for which suitable observations of [S III] exist (all with lower O/H than solar) have solar values of Ne/O and probably S/O and Ar/O, independent of O/H to within the errors, and N/O proportional (O/H)1.2 +/- 0.3. The chemical compositions of H II regions are compatible with those found in unevolved stars. These models do not address the He ionization, which is not well determined for cool exciting stars because the He0/He ratio depends on the He abundance assumed in the nebular model. For hot exciting stars, the standard He/H = He+ /H+ is correct.