Unveiling the birth and evolution of the HII region Sh2-173

TL;DR

This study investigates the origin and development of the HII region Sh2-173 through multiwavelength observations, revealing its young age, associated star formation, and potential hierarchical structure within the interstellar medium.

Contribution

The paper provides a detailed multiwavelength analysis of Sh2-173, including its age, stellar content, and its possible role within a hierarchical system of interstellar structures.

Findings

Sh2-173 is approximately 0.6 to 1.0 million years old.

A population of 46 YSO candidates was identified in the region.

Sh2-173 is likely part of a hierarchical system with a large HI shell.

Abstract

Based on a multiwavelength study, the ISM around the HII region Sh2-173 has been analyzed. The ionized region is clearly detected in the optical and in the radio continuum images. The analysis of the HI data shows a region of low emissivity that has an excellent morphological correlation with the radio continuum emission. The HII region is partially bordered by a photodissociation region, which, in turn, is encircled by a molecular structure. Taking into account the presence of noncircular motions in the Perseus spiral arm, together with previous distance estimates for the region, we adopt a distance of 2.5 +- 0.5 kpc for Sh2-173. Seven hot stars were identified in the field of Sh2-173, being only one an O-type star. The amount of energetic photons emitted by this star is enough to keep the region ionized and heat the dust. Given that an expanding HII region may trigger star formation, a search for YSO candidates was made using different infrared point source catalogues. A population of 46 YSO candidates was identified projected onto the molecular clouds. On the other hand, Sh2-173 is located in a dense edge of a large HI shell. The possibility for Sh2-173 of being part of a hierarchical system of three generations is suggested. The ages of both, the HII region and the large shell, were estimated and compared. We concluded that Sh2-173 is a young HII region of about 0.6 - 1.0 Myr old. As for the large shell we obtained a dynamical age of 5 +- 1 Myr. These age estimates, together with the relative location of the different structures, support the hypothesis that Sh2-173 is part of a hierarchical system.