Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions

Abstract

Synthetic integrated spectral properties of the old Galactic open clusters are studies in this work, in which 27 Galactic open clusters of ages ≥1 Gyr are selected as the working sample. Based on the photometric observations of these open clusters, a synthetic integrated spectrum has been made for the stellar population of each cluster. The effects of blue straggler (BS) stars on the conventional simple stellar population (SSP) model are analyzed on an individual cluster basis. It is shown that the BSs, whose positions in the color-magnitude diagrams cannot be predicted by the current single-star evolution theory, require significant modifications to the integrated properties of theoretical SSP model. The synthesized integrated spectral energy distributions (ISEDs) of our sample clusters are dramatically different from those of SSPs based on an isochrone only. The BS-corrected ISEDs of stellar populations show systematic enhancements toward shorter wavelengths in the spectra. When measured with broadband colors in unresolvable conditions, the age of a stellar population can be seriously underestimated by the conventional SSP model. Therefore, considering the common existence of BS components in real stellar populations, we should expect considerable alterations of the conventional ISEDs when we apply the technique of evolutionary population synthesis to more complicated stellar systems.