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

CICHOWOLSKI, S.; ROMERO, G.A.; ORTEGA, M.E.; CAPPA, C.E.; VASQUEZ, J.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

Año: 2008

0035-8711

Based on a multiwavelength study, the interstellar medium 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. The HI and CO structures related to Sh2-173 are observed in the velocity ranges from --25 to --31 km/s, and from --27 to --39 km/s, respectively. Taking into account the presence of non-circular motions in the Perseus spiral arm, together with previous distance estimates for the region, we adopt a distance of 25$\pm$0.5 kpc for Sh2-173. Seven hot stars were identified in the field of Sh2-173, being only one an O-type star. The amount of energetic photons emitted by this star is enough to keep the region ionized and heat the dust. Given that an expanding HII region may trigger star formation, a search for YSO candidates was made using different infrared point source catalogues. a population of 46 YSO candidates was identified projected onto the molecular clouds.On the other hand, sh2-173 is located in a dense edge of a large (~ 5deg) HI shell, GSH 117.8+1.5-35. The possibility for Sh2-173 of being part of a hierarchical system of three generations is suggested. In this scenario, the large HI shell, which was probably originated due to the action of Cas OB5, would have triggered the formation of Sh2-173, which, in turn, is triggering new stars in its surrounding molecular cloud. To test this hypothesis, the ages of both, the HII region and the large shell, were estimated and compared. We concluded that Sh2-173 is a young HII region of about 0.6-1.0 Myr old. as for the large shell we obtained a dynamical age of 5$\pm$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.