Theoretical H-Ii Region Models - The Effects Of Stellar Atmosphere Models

Photoionization modeling has been used to compute several grids of theoretical H II region models in order to assess the degree to which the choice of the ionizing stellar atmosphere model affects the calibration of the emission-line diagnostic diagrams of Evans & Dopita and the semiempirical H II region abundance sequence calibration of Dopita & Evans. Emission-line diagnostic diagrams are presented and compared for model nubulae ionized by Hummer & Mihalas unblanketed LTE atmospheres, Kurucz line-blanketed LTE atmospheres, Mihalas unblanketed non-LTE atmospheres, and a truncated blackbody spectrum. The models demonstrate that for solar nebular and atmospheric abundances, there are only minor differences between H II region models ionized by the Hummer & Mihalas atmospheres, and models ionized by the Kurucz atmospheres. The unblanketed non-LTE stellar atmosphere models of Mihalas and truncated blackbody spectra are shown to be unsuitable for general H II region modeling. Changing the atmospheric metallicity relative to the nebular metallicity has a significant effect on predicted emission-line ratios for model nebulae ionized by the Hummer & Mihalas atmospheres, but a considerably smaller effect for models ionized by the Kurucz atmospheres. Assuming that the atmospheric and nebular abundances are similar, it is estimated that the probable total errors in the semiempirical H II region abundance sequence calibration should not exceed +/- 0.15 dex for Z approximately 1/4Z., increasing to +/- 0.20 dex for Z approximately 2.5Z..