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

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. postal 70–264, Ciudad Universitaria, México DF 04510, México
² Laboratoire d'Astrophysique de Marseille, CNRS, BP 8, 13376 Marseille Cedex 12, France
³ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁴ Laboratoire d'Astrophysique, UMR 5572, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France

Corresponding author: C. Morisset, Morisset@AstroScu.UNAM.mx

Received: 3 June 2003
Accepted: 1 October 2003

Abstract

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 is correct these comparisons show the following: 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 dispersion plus additional dispersion of U.