Simple stellar population models as probed by the Large Magellanic Cloud star cluster ESO 121-SC03

Abstract

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 European Southern Observatory (ESO) 121-SC03, the only cluster populating the well-known 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 towards 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.Even using improved observational technologies, the number of galaxies with resolved stellar contents is limited. Comparison of the observed integrated spectral properties with models of 'simple' stellar populations (SSPs; single age, single-metallicity stellar populations), i.e. using the so-called population synthesis technique, is therefore a practical approach to studying the formation and the evolution processes in unresolved galaxies and their components. As the basic building blocks of the evolutionary population synthesis (EPS) method, conventional SSP models are generally constructed based on the evolution theory of single stars. (We specify the con-E-mail: y.xin@sheffield.ac.uk ventional SSP models throughout this paper as the theoretical SSP models based on single-star evolution theory.) Thus far, accuracy assessments of the existing suites of conventional SSP models have not yet adequately considered a potentially significant problem revealed by observations of star clusters.By assuming that all the original member stars within a cluster are born at the same time from the same progenitor molecular cloud, so that they have the same age and metallicity, star clusters are g...

A spectroscopic study of the blue stragglers in M67

“…In addition to their still elusive origin, the study of BSs is important because in a stellar population they are among the most massive and luminous stars, whose contribution to the integrated light cannot be predicted by the standard theory of stellar evolution (Bressan et al 1993). In fact, it has been demonstrated that they greatly affect the spectral energy distribution (SED) of the entire population (Manteiga, Martinez & Pickles 1989; Deng et al 1999), particularly at ultraviolet and blue wavelengths (Xin, Deng & Han 2007; Xin et al 2008).…”

The star clusters of the Magellanic Clouds

“…The effect of the blue stragglers is quite significant, especially blueward of ∼ 7000Å. These plots have been reproduced from Xin et al (2008). the resolved observations may be compared to the predictions of evolutionary synthesis models derived from, for example, the integrated cluster colours observed in different pass-bands.…”

Simple stellar population models including blue stragglers

“…Thus, as the fundamental test of our models, we compare our model results with the empirical conclusion from our previous work. Our main conclusion was that the BS‐induced colour changes are roughly 0.10 ± 0.12 mag in ( U − B ), 0.09 ± 0.09 mag in ( B − V ) and 0.05 ± 0.05 mag in ( V − R ), based on a working sample of Galactic OCs (Xin et al 2007) and the Large Magellanic Cloud (LMC) star cluster ESO 121–SC03 (Xin et al 2008, their table 1).…”