Theoretical models for H II regions. I - Diagnostic diagrams

Abstract

We have computed an extensive, homogeneous grid of theoretical photoionization models with conditions appropriate to observed H n regions. This grid has been used to develop a comprehensive set of theoretical diagnostic diagrams for H n regions, which employ ratios of prominent emission lines to determine the ionization parameter within the nebula, and the ionization temperature of its exciting star(s). It is also possible to estimate the element-averaged metallicity in the nebula and stars from these diagnostics. We construct theoretical loci for H n regions on the principal excitation diagrams of Baldwin, Phillips, and Terlevich (1981) and confirm that the theoretical models overlie the observed distribution of H n regions on these diagrams. Comparison of current observational data with the theoretical models on these and other diagrams implies the existence of a correlation between element-averaged metallicity and mean ionization parameter. The data also suggest that the ionization temperature of the exciting star(s) in the ionizing OB associations of H n regions is approximately constant and independent of metallicity and ionization parameter for the range of conditions considered here.