Star formation around RCW 120, the perfect bubble
  We 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 870
microns 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 and 70 microns to detect the young stellar
objects (YSOs) present in this region and to estimate their evolutionary
stages.
  A layer of dense neutral material surrounds the HII region, having been swept
up during the region's expansion. This layer has a mass greater than 2000 solar
masses and is fragmented, with massive fragments elongated along the ionization
front (IF). We measured the 24 microns flux of 138 sources. Of these, 39 are
Class I or flat-spectrum 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 microns core lies adjacent to the
IF. A 70 microns source with no 24 microns counterpart is detected at the same
position. This source is a likely candidate for a Class 0 YSO. Also at 24
microns, 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 microns
emission, the 24 microns emission, and the Halpha emission show the existence
of an extended and partially ionized photodissociation region around RCW 120.
