Blue Horizontal-Branch Stars and Simple Stellar Populations

The effects of blue horizontal-branch (HB) stars on the integrated spectrophotometric indices of simple stellar populations (SSPs) are addressed. Synthetic spectra are drawn from a grid of simulated globular clusters, constructed so as to reproduce the observed range of HB morphologies in the Milky Way system. Our models indicate that the Hbeta line index increases by as much as 0.75A and the integrated B-V colour becomes bluer by upwards of 0.15mag because of the contribution from blue HB stars. We discuss the importance of both optical and far-UV colours as age discriminants for SSPs, in particular for high metallicities. We suggest that relative age differences (and consequently their differing HB morphologies) result in non-linear colour-metallicity relations.