Spectrophotometric distances to Galactic H ii regions

Abstract

We present a near-infrared study of the stellar content of 35 H ii regions in the Galactic plane, 24 of which have been classified as giant H ii regions. We have selected these optically obscured star-forming regions from the catalogues of Russeil, Conti & Crowther and Bica et al. In this paper, we have used the near-infrared domain J-, H- and Ks-band colour images to visually inspect the sample. Also, we have used colour-colour and colour–magnitude diagrams to indicate ionizing star candidates, as well as the presence of young stellar objects such as classical T Tauri stars and massive young stellar objects (MYSOs). We have obtained Spitzer Infrared Array Camera images for each region to help further characterize them. Spitzer and near-infrared morphology were used to place each cluster in an evolutionary phase of development. Spitzer photometry was also used to classify the MYSOs. A comparison of the main sequence in the colour–magnitude diagrams for each observed cluster was used to infer whether or not the cluster kinematic distance is consistent with brightnesses of the stellar sources. We find qualitative agreement for a dozen of the regions, but about half the regions have near-infrared photometry that suggests they may be closer than the kinematic distance. A significant fraction of these already have spectrophotometric parallaxes that support smaller distances. These discrepancies between kinematic and spectrophotometric distances are not a result of the spectrophotometric methodologies, as independent non-kinematic measurements are in agreement with the spectrophotometric results. For instance, the trigonometric parallaxes of star-forming regions were collected from the literature and show the same effect of smaller distances when compared to the kinematic results. In our sample of H ii regions, most of the clusters are evident in the near-infrared images. Finally, it is possible to distinguish among qualitative evolutionary stages for these objects.