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

Extensive photoionization model grids for single star H  regions using a variety of recent state-of-the-art stellar atmosphere models have been computed with the main aim of constraining/testing their predicted ionizing spectra against recent ISO mid-IR observations of Galactic H  regions, which probe the ionizing spectra between ∼ 24 and 41 eV thanks to Ne, Ar, and S fine structure lines. Particular care has been paid to examining in detail the dependences of the nebular properties on the numerous nebular parameters (mean ionization parameter Ū, abundances, dust etc.) which are generally unconstrained for the objects considered here. Provided the ionization parameter is fairly constant on average and the atomic data is 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. On the current basis 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 which 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. – Interestingly, blackbodies reproduce best the observed excitation diagrams, which indicates that the ionizing spectra of our observed objects should have relative ionizing photon flux productions QE at energies 27.6, 35.0 and 41.1 eV close to that of blackbody spectra. These conclusions are found to be fairly robust to effects such as changes of Ū, nebular and stellar metallicity changes, and the inclusion of dust. Uncertainties due to atomic data (especially for Ar) are discussed. We also discuss the possibilities and difficulties in estimating absolute stellar temperatures from mid-IR line ratios, or softness parameters defined in analogy with the optical η indicator ηO−S=([O ] 3726,27Å/[O ] 4959,5007Å) / ([S ] 6717,31Å/[S ] 9069,9532Å). [Ar /] is found to be the only fairly stable Teff indicator. Mid-IR η’s appear to be of limited diagnostic power both empirically and theoretically. 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  regions, the actual contribution of this effect being finally atmosphere model dependent. We find that the dispersion of Teff between different H  regions due to statistical sampling of the IMF plus additional scatter in the ionization parameter are probably the dominant driver for the observed excitation scatter.