Journal Article

Unveiling the birth and evolution of the H <span class="smallCaps">ii</span> region Sh2-173

S. Cichowolski, G. A. Romero, M. E. Ortega, C. E. Cappa and J. Vasquez

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 2, pages 900-915
Published in print April 2009 | ISSN: 0035-8711
Published online March 2009 | e-ISSN: 1365-2966 | DOI:
Unveiling the birth and evolution of the H ii region Sh2-173

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Based on a multiwavelength study, the interstellar medium around the H ii region Sh2-173 has been analysed. The ionized region is clearly detected in the optical and the radio continuum images. The analysis of the H i data shows a region of low emissivity that has an excellent morphological correlation with the radio continuum emission. The H ii region is partially bordered by a photodissociation region, which, in turn, is encircled by a molecular structure. The H i and CO structures related to Sh2-173 are observed in the velocity ranges from −25 to −31 km s−1 and from −27 to −39 km s−1, 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 2.5 ± 0.5 kpc for Sh2-173. Seven hot stars were identified in the field of Sh2-173, being only one 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 H ii region may trigger star formation, a search for young stellar object (YSO) candidates was made using different infrared point source catalogues. A population of 46 YSO candidates was identified, projected on to the molecular clouds.

On the other hand, Sh2-173 is located in a dense edge of a large (∼5°) H i shell, GSH 117.8+1.5-35. The possibility of Sh2-173 being part of a hierarchical system of three generations is suggested. In this scenario, the large H i shell, which 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 H ii region and the large shell were estimated and compared. We concluded that Sh2-173 is a young H ii 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.

Keywords: stars: formation; H ii regions; ISM: individual: Sh2-173; ISM: kinematics and dynamics

Journal Article.  10090 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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