Observational study of sites of triggered star formation: CO and mid-infrared observations

Abstract

(Abridged) Bright-rimmed clouds (BRCs) are isolated molecular clouds located on the edges of evolved HII regions where star formation is thought may have been triggered. In this paper we investigate the current level of star formation within a sample of BRCs and evaluate to what extent star formation may have been induced. We present the results of a programme of position-switched CO observations towards 45 southern BRCs. The 12CO, 13CO and C18O (J=1-0) were simultaneously observed using the 22m Mopra telescope. We complement these observations with archival mid-IR submm and radio data. Analysis of the CO, mid-IR and radio data result in the clouds being divided into three distinct groups. We refer to these groups as spontaneous, triggered, and zapped clouds, respectively. Comparing the physical parameters of spontaneous and triggered samples we find striking differences in luminosity, surface temperature and column density with all three quantities significantly enhanced for the clouds considered to have been triggered. Furthermore, we find strong evidence for star formation within the triggered sample by way of methanol and H_2O masers, embedded mid-IR point sources and CO wings, however, we find evidence of ongoing star formation within only two of the spontaneous sample. We have used CO, mid-IR and radio data to identify 24 of the 45 southern BRCs that are undergoing a strong interaction with their HII region. We can therefore exclude ~50% from future studies. 14 of the 24 interacting BRCs are found to be associated with embedded mid-IR point sources and we find strong evidence of that these clouds are forming stars. The absence of mid-infrared sources towards the remaining ten clouds leads us to conclude that these represent an earlier evolutionary stage of star formation.