Predicted HST FOC and broad band colours for young and intermediate simple stellar populations

¹ Osservatorio Astronomico di Collurania, Via M. Maggini, I-64100 Teramo, Italy
² Istituto Nazionale di Fisica Nucleare, LNGS, I-67100 L'Aquila, Italy
³ Dipartimento di Fisica dell'Università di Pisa, Piazza Torricelli 2, I-56126 Pisa, Italy
⁴ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, U.S.A.
⁶ European Southern Observatory, K. Schwarzschild Str 2, Garching b. München D-85748, Germany e-mail: mromanie@eso.org

Send offprint request to: E. Brocato

Received: 6 July 1998
Accepted: 4 December 1998

Abstract

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 range of cluster ages from Myr to Gyr for three different metallicities (, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M - F307M ≤ 0.5 and for F170M - F278M ≤ 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration 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 combined use of HST UV colours and IR colours ( in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations ( Gyr).