Star Clusters in the Large Magellanic Cloud

Abstract

In this paper, we discuss the main properties of the family of star clusters in the Large Magellanic Cloud (LMC). To this aim. we first review the stellar models for low and intermediate mass stars in occurrence of convective overshoot, and then describe an algorithm suitable for a theoretical approach to cluster photometry (colour magnitude (C-M) diagrams. luminosity functions (LF). integrated magnitudes and colours, and total masses). The Johnson BV CCD photometry of the stellar content of NGC 1866 is presented and used to test the theoretical models for intermediate mass stars with convective overshoot. The conclusion of this analysis is that these models fit the C-M diagram and LF of NGC 1866 much better than any other type of model in literature. In addition to this, the Johnson BV CCD photometry of NGC 3 831, another populous cluster of LMC somewhat older than NGC 3 866, is presented and analyzed by means of the same stellar models. In particular, we discuss the effect of unresolved binary stars on alleviating the discrepancy between the age determined from the main sequence stars and the age determined from the red giant stars. Following an idea advanced by Eggen and Iben (1988, 1989) to explain the existence of blue stragglers in young disk aggregates, we explore the possibility that delayed star formation has taken place in NGC 1831. The final part of the review deals with the integrated photometry of star clusters. Theoretical calibrations between integrated colours and age as a function of the chemical composition are briefly discussed. The aim is to derive the age distribution function of LMC cluster and interpret the physical cause of their bimodal distribution in (B-V). The main result is that bimodality simply originates from the kind of age distribution that LMC clusters happen to possess, whereas all other explanations put forward in literature play a marginal role. Furthermore, it is argued that the age distribution function show two peaks of major activity of cluster formation, whose ages are centered at about 1.5×10 and 2×10 yr ago. We also present new estimates of the total mass of many clusters derived from their integrated BV magnitudes. Finally a few words are addressed to the issue of the history of star formation in the field stars of LMC.KeywordsStar FormationMain SequenceStar ClusterLuminosity FunctionStellar ModelThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.