Star formation around RCW 120, the perfect bubble

Context. This study deals with the star formation triggered by HII regions. Aims. 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. Methods. We present 870 mu m 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 24 mu m and 70 mu m to detect the young stellar objects present in this region and to estimate their evolutionary stages. Results. 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(circle dot) and is fragmented, with massive fragments elongated along the ionization front (IF). We measured the 24 mu m 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 870 mu m core lies adjacent to the IF. A 70 mu m source with no 24 mu m counterpart is detected at the same position. This source is a likely candidate for a Class 0 YSO. Also at 24 mu m, 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 mu m emission, the 24 mu m emission, and the H alpha 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.