Revisiting binary stars in population synthesis models

Abstract: We report results of a population synthesis model that follows the evolution of single and binary stars. In this model we include the 2HeWD merger channel, suggested by Han et al. (2002), for the formation of EHB stars. The physical parameters of the resulting EHB stars are derived from the BaSTI database, and are thus realistic and observationally supported. The predictions of this model are in good agreement with traditional population synthesis models, except when the spectrum of the stellar population is dominated by binary stars or their products, e.g., EHB stars in the UV of ETGs. We reproduce successfully the observed CMD and SED of the metal rich open cluster NGC 6791. The stellar population in this cluster may be archetypal of the stellar population in ETGs that show the UVX phenomenon. Our models should be appropriate to study the UV upturn in ETGs.