2006A&A...445..567B

We use J, H and K_S 2MASS photometry to study colour-magnitude (CMDs) and colour-colour diagrams, structure and mass distribution in the ionizing open cluster NGC 6611. Reddening variation throughout the cluster region is taken into account followed by field-star decontamination of the CMDs. Decontamination is also applied to derive the density profile and luminosity functions in the core, halo and overall (whole cluster) regions. The field-star decontamination showed that the lower limit of the main sequence (MS) occurs at ≃5M_☉. Based on the fraction of K_S excess stars in the colour-colour diagram we estimate an age of 1.3±0.3Myr which is consistent with the presence of a large number of pre-main sequence (PMS) stars. The distance from the Sun was estimated from known OV stars in the cluster area and the turn-on stars connecting the PMS and MS, resulting in d_☉=1.8±0.5kpc. The radial density distribution including MS and PMS stars is fitted by a King profile with a core radius R_core=0.70±0.08pc. The cluster density profile merges into the background at a limiting radius R_lim=6.5±0.5pc. From the field-star subtracted luminosity functions we derive the mass functions (MFs) in the form φ(m)∝m^–(1+χ). In the halo and through the whole cluster the MFs have slopes χ=1.52±0.13 and χ=1.45±0.12, respectively, thus slightly steeper than Salpeter's IMF. In the core the MF is flat, χ=0.62±0.16, indicating some degree of mass segregation since the cluster age is a factor ∼2 larger than the relaxation time. Because of the very young age of NGC 6611, part of this effect appears to be related to the molecular cloud-fragmentation process itself. We detect 362±120 PMS stars. The total observed mass including detected MS (in the range 5-85M_☉) and PMS stars amounts to ∼1600M_☉, thus more massive than the Trapezium cluster. Compared to older open clusters of different masses, the overall NGC 6611 fits in the relations involving structural and dynamical parameters. However, the core is atypical in the sense that it looks like an old/dynamically evolved core. Again, part of this effect must be linked to formation processes.