Age distribution of lmc clusters from their integrated ubv colors : history of star formation

Abstract

In this paper we revise the relationship between ages and metallicities of LMC star clusters and their integrated UBV colors. The study stands on the catalog of UBV colors of the Large Magellanic Cloud (LMC) clusters by Bica et al. (1994; BCDSP) and the photometric models of single stellar populations (SSP) calculated by Bertelli et al. (1994). These photometric models nicely describe the color distribution of LMC clusters in the (U – B) vs. (B – V) plane together with the observed dispersion of the colors and the existence of a gap in a certain region of this diagram. In the case of blue clusters, most of the dispersion in the colors can be accounted for by the presence of stochastic effects on the mass distribution of stars, whereas for the red ones additional dispersion's of ~ 0.2 dex in metallicity and of ~ 0.05 mag in color excess are needed. From comparing the observed distribution of integrated colors in the (U – B) vs. (B – V) diagram with the theoretical models, it turns out that: 1) The data are consistent with the presence of a gap (period of quiescence) in the history of cluster formation. If the age-metallicity relation (AMR) for the LMC obeys the simple model of chemical evolution, the gap is well evident and corresponds to the age interval ~ 3 Gyr to (12- 15) Gyr. On the contrary, if the chemical enrichment has been much slower than in the simple model, so that intermediate age clusters are less metal rich, the gap is expected to occur over a much narrower color range and to be hidden by effects of color dispersion. 2) The bimodal distribution of B – V colors can be reproduced by a sequence of clusters almost evenly distributed in the logarithm of the age, whose metallicity is governed by a normal AMR. No need is found of the so-called phase transitions in the integrated colors of a cluster taking place at suitable ages (Renzini & Buzzoni 1986). 3) The gap noticed by BCDSP in the (U – B) vs. (B – V) plane can be explained by the particular direction along which cluster colors are dispersed in that part of the (U – B) vs. (B – V) diagram. Also in this case, no sudden changes in the integrated properties of clusters must be invoked. The results of this analysis are used to revise the empirical method proposed by Elson & Fall (1985, EF85) to attribute ages to LMC clusters according to their integrated UBV colors. We show that the EF85 method does not provide the correct relation between ages and colors for clusters of low metallicity and hence its inability to date the old clusters. We propose two modifications to the definition of the parameter 8 of EF85 such that the age sequence of red clusters is suitably described, and the intrinsic errors on ages caused by the heavy presence of various effects dispersing the colors are reduced to a minimum. The age sequence is calibrated on 24 template clusters for which ages were independently derived from recent color-magnitude diagrams (CMD). Finally, we attribute ages to all clusters present in BCDSP catalog, and derive the global age distribution function (ADF) for LMC clusters. The ADF presents new features that were not clear in previous analyses of UBV data, but were already suggested by a number of independent observational studies. The features in question are periods of enhanced cluster formation at ~100 Myr and 1 – 2 Gyr, and a gap in the cluster formation history between ~ 3 and (12 – 15) Gyr. The peaks observed in the distribution of B – V colors are found to be sensítíve to the presence of these periods of enhanced cluster formatíon and the lack of extremely red clusters caused by the age gap between íntermediate-age and old clusters.