Mid-IR observations of Galactic HII regions: Constraining ionizing spectra of massive stars and the nature of the observed excitation sequences

Extensive photoionization model grids for single star H II regions using state-of-the-art stellar atmosphere models have been computed to test their predicted ionizing spectra against recent ISO mid-IR observations of Galactic H II regions. Particular care has been paid to examining in detail the dependences of the nebular properties on the numerous nebular parameters which are generally unconstrained. Provided the ionization parameter U is fairly constant on average and the atomic data correct these comparisons show the following: - Both recent non-LTE codes including line blanketing and stellar winds (WM-Basic and CMFGEN) show a reasonable agreement with the observations, although non-negligible differences between their predicted ionizing spectra are found. Recurrently none of the models can be preferred over the other. -The softening of the ionizing spectra with increasing metallicity predicted by the WM-Basic models is found to be too strong. -We confirm earlier indications that the CoStar atmospheres, including an approximate treatment of line blanketing, overpredict somewhat the ionizing flux at high energies. -Both LTE and non-LTE plane parallel hydrostatic atmosphere codes predict ionizing spectra that are too soft, especially over the energy range between 27.6, 35.0, and 41.1 eV and above. The inclusion of wind effects is crucial for accurate predictions of ionizing fluxes. These conclusions are found to be robust to effects such as changes of U, stellar metallicity changes, and the inclusion of dust. Uncertainties due to atomic data (especially for Ar) are discussed. We also discuss the difficulties in estimating absolute stellar temperatures from mid-IR line ratios. Finally we have examined which parameters are chiefly responsible for the observed mid-IR excitation sequences. The galactic gradient of metallicity changing the shape of the stellar emission is found to be one of the drivers for the excitation sequence of Galactic H II regions, the actual contribution of this effect being finally atmosphere model dependent. The observed excitation scatter can be explained by effects due to statistical sampling of the IMF leading to a T-eff dispersion plus additional dispersion of U.