Abstract
Several grand-design spiral galaxies show a bimodal distribution of their system of star clusters and star forming complexes in JHK diagrams. The (J-H) vs (H-K_{s}) diagram revealed that embedded clusters, still immersed in their parental clouds of gas and dust, have in general a redder (H-K_{s}) colour than older clusters, whose gas and dust have already been ejected. In addition, the reddening-free index Q_{d}=(H-K_{s})-0.884 (J-H) was shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. In the present work, the aforementioned photometric indices were explored for star cluster systems in the Local Group. In particular, we investigate the effectiveness of the Q_{d} index in sorting out clusters of different ages at their early evolutionary stages. Surface photometry on 2MASS images was carried out for populous clusters younger than approximately 100 Myr and whose ages are available. The integrated magnitudes and colors extracted from the surface photometry showed the same bimodal distribution in JHK diagrams as that found for more distant galaxies, suggesting that the phenomenon is universal. We confirm the index Q_{d} as a powerful tool to distinguish clusters younger than about 7 Myr from older clusters.
