The physical structure of Magellanic Cloud HII regions - II. Elemental abundances

Based on a new data set of optical and infrared spectra described in Vermeij et al. (2001), we analyse the gas-phase elemental abundances of a sample of H II regions in the Large and Small Magellanic Cloud. The combined optical and infrared data set gives us access to all the ionization stages of astrophysically important elements such as sulfur and oxygen. We self-consistently determine the electron temperatures and densities for the O+, S++ and O++ ionization zones, and use these parameters in the derivation of the ionic fractions. We discuss the uncertainties on these ionic fractions. The different relations between the electron temperatures as proposed by Garnett (1992) and Thuan et al. (1995) are confronted with our results. We find our electron temperatures to be consistent with these relations, although the relation between T-e [S III] and T-e [O III] might be slightly steeper than predicted. We investigate the reliability of the Ionization Correction Factors (ICFs) used in the derivation of the full elemental abundances of sulfur and neon. We conclude that the prescription for the ICF used to derive the sulfur abundance as given by Stasinska (1978) for alpha = 3 is accurate for O+/O> 0.20. No conclusions could be drawn for neon. Avoiding the use of ICFs as much as possible, we then proceed to derive the full elemental abundances. We calculate a grid of general photoionization models to compare our results with the bright-line abundance diagnostics for oxygen (R23) and sulfur (S23(4)). The reliability of the newly proposed S234 parameter (Oey & Shields 2000) which includes emission lines from S+, S++ and S+3 is checked. We find a very good agreement between the S234 models and our analysis results. Finally, we compare the heavy element-to-oxygen ratios of our sample objects to those of giant H II regions in a large sample of low-metallicity blue dwarf galaxies (Izotov & Thuan 1999) and with the results from Kobulnicky & Skillman (1996, 1997) for the irregular galaxies NGC 1569 and NGC 4214.