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

Abstract

Abstract. Extensive photoionization model grids for single star H  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  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 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. 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  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.

The Physical Conditions in Starbursts Derived From Bayesian Fitting of Mid-Infrared Spectral Energy Distribution Models: 30 Doradus as a Template

“…However, the Levesque et al (2010) models use a much simpler geometry (namely plane-parallel instead of spherical), demonstrating much more clearly how the degeneracy between the hardness of the radiation field (i.e., stellar cluster age) and the ionization parameter, Q (the ratio of the ionizing photon density to gas density), is broken using four strong mid-infrared emission lines. The dependence of the line ratios on these two parameters has also been noted by Morisset et al (2004). Our comparison between the measured line ratios and the predictions from the Levesque models indicates ages between 2.0 and 2.5 Myr for all four positions.…”

A quantitative study of O stars in NGC 2244 and the Monoceros OB2 association

“…generally consistent with what is required to ionize the nebulae (Morisset et al 2004;Simón-Díaz & Stasińska 2008); -CMFGEN does not compute the wind properties from first principles (see Sect. 3).…”

Fundamental parameters of B supergiants from the BCD system

“…Thus, both spectral lines and the emitted continuum energy distribution can in principle be modified (Abbott & Hummer 1985;Gabler et al 1989;Smith et al 2002;Morisset et al 2004). Since the models used in this work are wind-free, doubts can be raised about whether the use of these models introduces systematic effects on the estimate of T f eff .…”

“…For the obtained T eff they yield somewhat larger Balmer discontinuities than those observed. This might be partially due to the planeparallel approximation of model atmospheres that probably is not suited to the extended atmospheric layers of these stars, to an incomplete treatment of the spectral line formation in such diluted atmospheres, to the omission of the effects produced on the photosphere by the stellar winds (Abbott & Hummer 1985;Gabler et al 1989;Smith et al 2002;Morisset et al 2004), and also to an insufficient line blocking in the model atmospheres, as discussed by Remie & Lamers (1981). Thus, to obtain the T f eff parameter of supergiants, we have slightly modified the use of relations (1) and (2) through the following iteration procedure: 1) adopt approximate values of T eff and log g; 2) interpolate the model fluxes in the far-UV and IR for the adopted (T eff , log g) parameters;…”