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

Abstract

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 Teff of the exciting O stars and the mean ionization parameter U, where Teff and U are found to increase and decrease, respectively, with the metallicity of the H II region represented by the Ne/Nesolar ratio. No evidence is found for gradients of Teff or U with Galactocentric distance RGal. The observed excitation sequence with RGal is mainly due to the effect of the metallicity gradient on the spectral ionizing shape, upon which the effect of an increase in Teff 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 Teff with Z and an increase of Teff with RGal.