Spectrophotometric distances to Galactic H ii regions

Abstract

We present a near infrared study of the stellar content of 35 H\\,{\\sc{ii}}\nregions in the Galactic plane. In this work, we have used the near infrared\ndomain $J-$, $H-$ and $K_{s}-$ band color images to visually inspect the\nsample. Also, color-color and color-magnitude diagrams were used to indicate\nionizing star candidates, as well as, the presence of young stellar objects\nsuch as classical TTauri Stars (CTTS) and massive young stellar objects\n(MYSOs). We have obtained {\\it Spitzer} IRAC images for each region to help\nfurther characterize them. {\\it Spitzer} and near infrared morphology to place\neach cluster in an evolutionary phase of development. {\\it Spitzer} photometry\nwas also used to classify the MYSOs. Comparison of the main sequence in\ncolor-magnitude diagrams to each observed cluster was used to infer whether or\nnot the cluster kinematic distance is consistent with brightnesses of the\nstellar sources. We find qualitative agreement for a dozen of the regions, but\nabout half the regions have near infrared photometry that suggests they may be\ncloser than the kinematic distance. A significant fraction of these already\nhave spectrophotometric parallaxes which support smaller distances. These\ndiscrepancies between kinematic and spectrophotometric distances are not due to\nthe spectrophotometric methodologies, since independent non-kinematic\nmeasurements are in agreement with the spectrophotometric results. For\ninstance, trigonometric parallaxes of star-forming regions were collected from\nthe literature and show the same effect of smaller distances when compared to\nthe kinematic results. In our sample of H\\,{\\sc{ii}} regions, most of the\nclusters are evident in the near infrared images. Finally, it is possible to\ndistinguish among qualitative evolutionary stages for these objects.\n