Blue Stragglers in Clusters and Integrated Spectral Properties of Stellar Populations

Abstract

Evolutionary population synthesis (EPS) has been widely used as a powerful tool to study the stellar contents of galaxies. In essence, EPS compares the observed, integrated spectrum of a galaxy with a combination of spectra of simple stellar populations (SSPs; single-age, single-metallicity populations) of different ages and metallicities, and decomposes the complex stellar contents into SSPs of known ages and metallicities to infer the galaxy's star-formation history. Over the past two decades, much work has been done to improve the accuracy of EPS and SSP models in various contexts (e.g., [6,7,19,34,35,36]).Unfortunately, population synthesis models still suffer from a number of limitations. One is our poor understanding of some advanced single-star evolutionary phases, such as of supergiants and asymptotic giant branch stars [41], while a second is an absence in the models of the results of stellar interactions, such as the so-called "stragglers" formed through mass transfer in binaries or stellar collisions. Such stars are usually very bright and can strongly affect the integrated-light properties of the entire system. The potential uncertainties inherent to EPS caused by ignoring these components could be much larger than those still remaining and due to the variety of input physics among different models.In this chapter, we focus on the second limitation to the standard SSP models. With the updated knowledge of stars, in particular on the physical properties of binary and collisional interactions that eventually create the exotic blue straggler stars (BSSs) discussed in previous chapters in this book, a more realistic prescription of stellar populations is now possible. The goal of this chapter is to present a new set of SSP models which include contributions from BSSs.