Colour evolution models and the distribution of lmc clusters in the integrated ubv plane

Abstract

We present a comparison between photometric cluster models, based on classical and with-overshooting stellar tracks, and the enlarged sample of 624 LMC clusters recently gathered in integrated UBV photometry by Bica et al. Models based on Maeder and Meynet's tracks present two temporary red phases: the first at age 10 Myr, caused by a clump of red supergiants; the second at ~ 100 Myr due to the combined effect of both the progressive reduction of the blue loop of core Heburning stars, and their fading relative to top-MS stars. The 100 Myr red phase does not occur in models without overshooting. Taking into account stochastic effects on the mass distribution of stars, the models describe well the general distribution of clusters in the (U – B) vs. (B – V) diagram, except for the oldest, SWB types V-VII, clusters. The dispersion of cluster colours due to stochastic effects is found to be strongly variable along the ageing sequence: the general trend is a decrease with age due to the increasing population of post-MS phases, but the dispersion increases in the temporary red phases and is expected to increase again after the red giant branch phase transition due to the appearance of extended RGBs and carbon stars. We also study the LMC clusters age distribution function, based on the age frequency of clusters of equal initial masses, taking into account different values for the IMF slope.