Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model

The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of simple stellar population (SSP) models. These are based on the stellar evolution theory of single stars, whereas the major formation mechanisms of BSs are all correlated with stellar interactions. We have illustrated this in a previous study based on a small sample of old (age >=1 Gyr) Galactic OCs. However, for the purpose of demonstrating the contributions of BSs to the conventional SSP models statistically and systematically, a large database with sufficient coverage of age and metallicity is definitely needed. The working sample now includes 100 Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster basis. The general existence of BSs in our star cluster sample dramatically alters the predictions of conventional SSP models in terms of their integrated properties. Neglecting the consequences of nonstandard evolutionary products, such as BSs, in stellar populations, very large uncertainties can be made in analyzing their integrated spectral energy distributions at unresolvable conditions. The current work strongly suggests that when evolutionary population synthesis technique is used to study the properties of unresolved stellar populations in galaxies, the contributions of BSs should be taken into account.