Predicted HST FOC and broad band colours for young and intermediate simplestellar populations

This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the ra nge of cluster ages from t = 8 Myr to t = 5 Gyr for three different metalli cities (Z = 0.02, 0.006, and 0.001). Statistical fluctuations in the cluste r population have been investigated, assessing the predicted fluctuations o f the various colours as a function of the cluster integrated absolute magn itude. We show that the red leak in HST UV filters deeply affects the predi cted fluxes and colours. However, we find that for F152M - F307M less than or equal to 0.5 and for F170M - F278M less than or equal to 0.5 (which mean s ages lower than 1 Gyr) the HST UV colours can still be used to infer reli able indications on the age of distant clusters. Moreover, one finds that t he age calibration of these colours is scarcely affected by the amount of o riginal helium or by the assumed IMF. On this basis, we present a calibrati on of the HST UV two-colours (F152M - F307M vs. F170M - F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combi ned use of HST UV colours and IR colours (V - K in particular) to disentang le the metallicity-age effect in integrated colours of young stellar popula tions (t less than or equal to 1 Gyr).