Spectrophotometric Distances to Galactic H\,{\sc{ii}} Regions

TL;DR

This study uses near-infrared and Spitzer data to analyze 35 Galactic HII regions, revealing their stellar content, evolutionary stages, and discrepancies between kinematic and spectrophotometric distances, with implications for understanding star formation.

Contribution

It provides a comprehensive near-infrared analysis of HII regions, combining multiple data sources to assess distances and evolutionary stages, highlighting discrepancies with kinematic measurements.

Findings

Approximately half of the regions have closer distances than kinematic estimates.

Spectrophotometric and trigonometric parallaxes support smaller distances.

Most clusters are visible in near-infrared images and can be classified by evolutionary stage.

Abstract

We present a near infrared study of the stellar content of 35 H\,{\sc{ii}} regions in the Galactic plane. In this work, we have used the near infrared domain $J-$, $H-$ and $K_{s}-$ band color images to visually inspect the sample. Also, color-color and color-magnitude diagrams were used to indicate ionizing star candidates, as well as, the presence of young stellar objects such as classical TTauri Stars (CTTS) and massive young stellar objects (MYSOs). We have obtained {\it Spitzer} IRAC images for each region to help further characterize them. {\it Spitzer} and near infrared morphology to place each cluster in an evolutionary phase of development. {\it Spitzer} photometry was also used to classify the MYSOs. Comparison of the main sequence in color-magnitude diagrams to 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 which support smaller distances. These discrepancies between kinematic and spectrophotometric distances are not due to the spectrophotometric methodologies, since independent non-kinematic measurements are in agreement with the spectrophotometric results. For instance, 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\,{\sc{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.