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...
By using synthetic integrated spectral energy distribution (ISED) of open clusters and a new evolutionary scenario involving interactive binaries, the theoretical model of simple stellar population (SSP) models are tested. Blue stragglers stars are commonly present in open clusters, which are usually around 2 magnitudes brighter and much bluer than the MS turn-off of the cluster, and they certainly belong to the same population as the host cluster. Considering the cluster as the nearest counterpart the concept of SSP, inclusion of such stars may have significant consequences on the integrated of properties of the population. We are showing in the present work that if one is to fit the real stellar population using the classical model based only on single star evolution theory, large errors in either age or metallicity (or both) can be made. The new theoretical SSP incorporated with interactive binaries can explain part of the difference between classical SSP model and synthetic ISED built up with open clusters. This suggests that the formation of blue stragglers in real clusters is more complicated as usually assumed in binary stellar population synthesis.Assembly of simple stellar populations (SSPs) is a common practice to study stellar content in complicated stellar systems especially at unresolvable situations (Bressan, Chiosi & Fagotto 1994, Bruzual & Chalot 2003. The very basic ingredients of modeling galaxies, the theoretical SSP models (referred as convectional SSP), can be tested using star clusters. A star cluster is the nearest counterpart of the concept of SSP model, the most prominent difference between them is actually due to the bright struggling stars in the observed CMD. The minor difference would be the low mass stars evaporated away from the cluster during the past history of dynamical evolution. The bright components of a star cluster, being the massive members of the system, do not suffer such problem, therefore they should reflect the initial and present content of the massive stars in the cluster. The ISED of present day stellar content of a star cluster, after correction of these missing low mass stars, is then a very good test against the theoretical SSP. We have carried this test by building up the synthetic ISED of open clusters (Deng et al. 1999, Xin & Deng 2005, Xin, Deng & Han 2006. SSP models with binary interactions have also been available (Zhang, Li & Han 2005).Assuming star clusters, or binary stellar populations (BSPs) as the true SSPs, and fitting them using the conventional SSP models in terms of ISEDs, the deviations can be regarded as the uncertainties that are intrinsic in the conventional SSP models in understanding the nature of stellar populations at unresolved conditions. The difference between the fittings using clusters and BSPs actually reveals the physical processes that are different from what have been considered in the BSPs.Given the gravitationally evaporated low mass stars are retrieved, open clusters can be regarded as simple Stellar Populations. All stars, if...
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