2009A&A...496..177D

This study deals with the star formation triggered by Hii regions. We wish to take advantage of the very simple morphology of RCW 120 - a perfect bubble - to understand the mechanisms triggering star formation around an Hii region and to establish what kind of stars are formed there. We present 870µm observations of RCW 120, obtained with the APEX-LABOCA camera. These show the distribution of cold dust, and thus of neutral material. We use Spitzer-MIPS observations at 24µm and 70µm to detect the young stellar objects present in this region and to estimate their evolutionary stages. A layer of dense neutral material surrounds the entire HII region, having been swept up during the region's expansion. This layer has a mass greater than 2000 M_☉ and is fragmented, with massive fragments elongated along the ionization front (IF). We measured the 24µm flux of 138 sources. Of these, 39 are Class I or flat-spectrum young stellar objects (YSOs) observed in the direction of the collected layer. We show that several triggering mechanisms are acting simultaneously in the swept-up shell, where they form a second generation of stars. No massive YSOs are detected. However, a massive, compact 870µm core lies adjacent to the IF. A 70µm source with no 24µm counterpart is detected at the same position. This source is a likely candidate for a Class 0 YSO. Also at 24µm, we detect a chain of about ten regularly spaced Class I or flat spectrum sources, parallel to the IF, in the direction of the most massive fragment. We suggest that the formation of these YSOs is the result of Jeans gravitational instabilities in the collected layer. Finally, the 870 µm emission, the 24 µm emission, and the Hα emission show the existence of an extended and partially ionized photodissociation region around RCW 120. This demonstrates the long-distance influence of the HII region upon its surrounding medium.