A close look at the heart of RCW 108 ^*,**

¹ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: fcomeron@eso.org
² SAp/CEA Saclay, 91191 Gif-sur-Yvette, France e-mail: nschneid@cea.fr

Received: 27 April 2007
Accepted: 4 July 2007

Abstract

Context.The IRAS 16362-4845 star-forming site in the RCW 108 complex contains an embedded compact cluster that includes some massive O-type stars. Star formation in the complex, and in particular in IRAS 16362-4845, has been proposed to be externally triggered by the action of NGC 6193.

Aims.We present a photometric study of the IRAS 16362-4845 cluster sensitive enough to probe the massive brown dwarf regime. In particular, we try to verify an apparent scarcity of solar-type and low-mass stars reported in a previous paper (Comerón et al. 2005, A&A, 433, 955).

Methods.Using NACO at the VLT we have carried out adaptive optics-assisted imaging in the bands, as well as through narrow-band filters centered on the Brγ and the H₂  lines. We estimate individual line-of-sight extinctions and, for stars detected in the three filters, we estimate the contribution to the K_S flux caused by light reprocessed in the circumstellar environment. We also resolve close binary and multiple systems. We use the K luminosity function as a diagnostic tool for the characteristics of the underlying mass function.

Results.IRAS 16362-4845 does contain young low-mass stars. Nevertheless, they are far less than those expected from the extrapolation of the bright end of the K luminosity function towards fainter magnitudes. We estimate a total stellar mass of 370 . Nearly all the cluster members display L' excesses, whereas K_S excesses are in general either absent or moderate (<1 mag). We also detect an extremely red object with , likely to be a Class I source.

Conclusions.The fact that solar-type and low-mass stars are present in numbers much smaller than those expected from the number of more massive members hints at an initial mass function deficient in low mass stars as compared to that of other young clusters such as the Trapezium. The origin of this difference is unclear, and we speculate that it might be due to external triggering having started star formation in the cluster, perhaps producing a top-heavy initial mass function. We also note that there are no detectable systematic differences between the spatial distributions of bright and faint cluster members. Such absence of mass segregation in the spatial distribution of stars may also support external triggering having played an important role in the history of the RCW 108 region.