Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121–SC03

The presence of blue straggler stars (BSs) in star clusters has proven a challenge to conventional simple stellar population (SSP) models. Conventional SSP models are based on the evolution theory of single stars. Meanwhile, the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main sequence turn-off point. Such loci cannot be predicted by single-star evolution theory. However, stars with such properties contribute significantly to the integrated light of the cluster. In this paper, we reconstruct the integrated properties of the Large Magellanic Cloud cluster ESO 121–SC03, the only cluster populating the wellknown age gap in the cluster age distribution, based on a detailed exploration of the individual cluster stars, and with particular emphasis on the cluster’s BSs. We find that the integrated light properties of ESO 121–SC03 are dramatically modified by its BS component. The integrated spectral energy distribution (ISED) flux level is significantly enhanced toward shorter wavelengths, and all broad-band colours become bluer. When fitting the fully integrated ISED of this cluster based on conventional SSP models, the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters. The age underestimate is ∼ 40 per cent if we only include the BSs within the cluster’s half-light radius and ∼ 60 per cent if all BSs are included. The corresponding underestimates of the cluster’s metallicity are ∼ 30 and ∼ 60 per cent, respectively. The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner, since they cover a large range in age and metallicity.