Star formation in RCW 108: triggered or spontaneous?

Abstract

We present visible, near IR, and mm-wave observations of RCW 108, a molecular cloud complex in the AraOB1 association that is being eroded by the energetic radiation of two O-type stars in the nearby cluster NGC 6193. The western part of the RCW108 cloud contains an embedded compact HII region, IRAS 16362-4845, ionized by an aggregate of early-type stars. We notice a lack of stars later than A0 in the aggregate and speculate that this might be a consequence of its extreme youth. We examine the distribution of stars displaying IR excesses projected across the molecular cloud. While many of them are located in the densest area of the molecular cloud near IRAS16362-4845, we also find a group concentrating towards the edge of the cloud that faces NGC 6193, as well as some other stars beyond the edge of the molecular cloud. The intense ionizing radiation field by the O stars in NGC6193 is a clear candidate trigger of star formation in the molecular cloud, and we suggest that the existence and arrangement of stars in this region of the cloud supports a scenario in which their formation may be a consequence of this. However, IR excess stars are also present in some areas of the opposite side of the cloud, where no obvious candidate external trigger is identified. The existence of such tracers of recent star formation scattered across the more massive molecular cloud associated with IRAS 16362-4845, and the low star formation efficiency that we derive, indicate that it is in a state to still form stars. This is in contrast to the less massive cloud close to NGC 6193, which seems to be more evolved and mostly already recycled into stars, and whose internal kinematics show hints of having been perturbed by the presence of the massive stars formed out of it.