O Star Effective Temperatures and H II Region Ionization Parameter Gradients in the Galaxy

Extensive photoionization model grids are computed for single-star H II regions using stellar atmosphere models from the WM-Basic code. Mid-IR emission-line intensities are predicted, and diagnostic diagrams of [Ne III/II] and [S IV/III] excitation ratios are built, taking into account the metallicities of both the star and the H II region. The diagrams are used in conjunction with Galactic H II region observations obtained with the Infrared Space Observatory to determine the effective temperature T_eff of the exciting O stars and the mean ionization parameter , where T_eff and are found to increase and decrease, respectively, with the metallicity of the H II region represented by the Ne/Ne_☉ ratio. No evidence is found for gradients of T_eff or with Galactocentric distance R_Gal. The observed excitation sequence with R_Gal is mainly due to the effect of the metallicity gradient on the spectral ionizing shape, upon which the effect of an increase in T_eff with Z is superposed. We show that not taking properly into account the effect of metallicity on the ionizing shape of the stellar atmosphere would lead to an apparent decrease of T_eff with Z and an increase of T_eff with R_Gal.