Triggered star formation on the borders of the Galactic HII region RCW 82

Context. We are engaged in a multi-wavelength study of several Galactic HII regions that exhibit signposts of triggered star formation on their borders, where the collect and collapse process could be at work. Aims. When addressing the question of triggered star formation, it is critical to ensure the real association between the ionized gas and the neutral material observed nearby. In this paper we stress this point and present CO observations of the RCW 82 star forming region. Methods. The velocity distribution of the molecular gas is combined with the study of young stellar objects (YSOs) detected in the direction of RCW 82. We discuss the YSO's evolutionary status using near- and mid-IR data. The spatial and velocity distributions of the molecular gas are used to discuss the possible scenarios for the star formation around RCW 82. Results. Several massive molecular condensations, together with star formation sites, are observed on the borders of RCW 82. The shapes of the three brightest condensations suggest that they were pre-existent, i.e. not formed through the collect and collapse process. A thin layer of molecular material is observed surrounding the ionized gas, adjacent to the ionization front. This results from the sweeping up of neutral material around the expanding region. Several Class I YSOs are detected in the direction of this layer. Conclusions. The numerous YSOs observed towards the bright molecular condensations bordering (and velocity-associated with) the ionized gas reveal the intense star formation activity in RCW 82. But this region is probably too young to have triggered star formation via the collect and collapse process.