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Star formation around RCW 120, the perfect bubble
Context: This study deals with the star formation triggered by H iiregions. Aims: We wish to take advantage of the very simplemorphology of RCW 120 - a perfect bubble - to understand the mechanismstriggering star formation around an H ii region and to establish whatkind of stars are formed there. Methods: We present 870 ?mobservations of RCW 120, obtained with the APEX-LABOCA camera. Theseshow the distribution of cold dust, and thus of neutral material. We useSpitzer-MIPS observations at 24 ?m and 70 ?m to detect the youngstellar objects present in this region and to estimate theirevolutionary stages. Results: A layer of dense neutral materialsurrounds the entire H ii region, having been swept up during theregion's expansion. This layer has a mass greater than2000~M? and is fragmented, with massive fragmentselongated along the ionization front (IF). We measured the 24 ?m fluxof 138 sources. Of these, 39 are Class I or flat-spectrum young stellarobjects (YSOs) observed in the direction of the collected layer. We showthat several triggering mechanisms are acting simultaneously in theswept-up shell, where they form a second generation of stars. No massiveYSOs are detected. However, a massive, compact 870 ?m core liesadjacent to the IF. A 70 ?m source with no 24 ?m counterpart isdetected at the same position. This source is a likely candidate for aClass 0 YSO. Also at 24 ?m, we detect a chain of about ten regularlyspaced Class I or flat spectrum sources, parallel to the IF, in thedirection of the most massive fragment. We suggest that the formation ofthese YSOs is the result of Jeans gravitational instabilities in thecollected layer. Finally, the 870 ?m emission, the 24 ?m emission,and the H? emission show the existence of an extended andpartially ionized photodissociation region around RCW 120. Thisdemonstrates the long-distance influence of the H ii region upon itssurrounding medium.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/496/177
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