Star formation in RCW 108: Triggered or spontaneous?

Abstract

We present visible, near infrared and mm-wave observations of RCW 108, a molecular cloud complex in the Ara OB1 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 molecular cloud, for which we derive a mass of ~8000 $M_\odot$, contains an embedded compact HII region, IRAS 16362-4845, ionized by an aggregate of early-type stars for which we estimate a mass of ∼210 $M_\odot$. The spectral type of the earliest star is O9, as confirmed by the visible spectrum of the compact HII region. We notice a lack of stars later than A0 in the aggregate, at least having the moderate reddenings that are common among its B-type stars, and we speculate that this might be a consequence of the extreme youth of the aggregate. We also note the existence of a dense ionized clump ($n > 10^4$ cm-3) appearing near the main ionizing star of the compact HII region. We examine the distribution of stars displaying infrared excesses projected across the molecular cloud. While many of them are located in the densest ($n \sim 10^{4-5}$ cm-3) area of the molecular cloud near the position of IRAS 16362-4845, 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 NGC 6193 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 molecular cloud supports a scenario in which their formation may be a consequence of this. However, infrared 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 (∼660 $M_\odot$) 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.