Simple stellar population models including blue stragglers

Abstract

Observations show that nearly all star clusters and stellar populations contain blue straggler stars (BSs). BSs in a cluster can significantly enhance the integrated spectrum of the host population, preferentially at short wavelengths, and render it much bluer in photometric colours. Current theoretical simple stellar population (SSP) models constructed within the traditional framework of single and binary stellar evolution cannot fully account for the impact of these objects on the integrated spectral properties of stellar populations. Using conventional SSP models without taking into account BS contributions may significantly underestimate a cluster's age and/or metallicity, simply because one has to balance the observed bluer colours (or a bluer spectrum) with a younger age and/or a lower metallicity. Therefore, inclusion of BS contributions in SSP models is an important and necessary improvement for population synthesis and its applications. Here, we present a new set of SSP models, which include BS contributions based on our analysis of individual star clusters. The models cover the wavelength range from 91~Å to 160~$μ$m, ages from 0.1 to 20 Gyr and metallicities $Z=0.0004, 0.004, 0.008, 0.02$ (solar) and 0.05. We use the observed integrated spectra of several Magellanic Cloud star clusters to cross-check and validate our models. The results show that the age predictions from our models are closer to those from isochrone fitting in the clusters' colour-magnitude diagrams compared to age predictions based on standard SSP models.