The infrared dust bubble N22: an expanding H ii region and the star formation around it

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
e-mail: jiweiguang@xao.ac.cn
² Graduate University of the Chinese Academy of Sciences, 100080 Beijing, PR China
³ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Department of Astronomy, Peking University, 100871 Beijing, PR China

Received: 21 January 2012
Accepted: 30 May 2012

Abstract

Aims. To increase the observational samples of star formation around expanding H ii regions, we analyzed the interstellar medium and star formation around N22.

Methods. We used data extracted from the seven large-scale surveys from infrared to radio wavelengths. In addition we used the JCMT^⋆ observations of the J = 3−2 line of ¹²CO emission data released on CADC^⋆⋆ and the ¹²CO J = 2−1 and J = 3−2 lines observed by the KOSMA^⋆⋆⋆ 3 m telescope. We performed a multiwavelength study of bubble N22.

Results. A molecular shell composed of several clumps agrees very well with the border of N22, suggesting that its expansion is collecting the surrounding material. The high integrated ¹²CO line intensity ratio R_{ICO(3 − 2)/ICO(2 − 1)} (ranging from 0.7 to 1.14) implies that shocks have driven into the molecular clouds. We identify eleven possible O-type stars inside the H ii region, five of which are located in projection inside the cavity of the 20 cm radio continuum emission and are probably the exciting-star candidates of N22. Twenty-nine young stellar objects (YSOs) are distributed close to the dense cores of N22. We conclude that star formation is indeed active around N22; the formation of most of YSOs may have been triggered by the expanding of the H ii region. After comparing the dynamical age of N22 and the fragmentation time of the molecular shell, we suggest that radiation-driven compression of pre-existing dense clumps may be ongoing.