Ultraviolet ages of young clusters in the Magellanic Clouds.

Abstract

Following a previous investigation on the integrated UV colours of stellar clusters (Barbero et al. 1990), we study the calibration of the ultraviolet colour index C(15-31) in terms of cluster age, using observations by the International Ultraviolet Explorer of 29 young and populous clusters of the Large Magellanic Cloud (LMC), and of the Small Magellanic Cloud (SMC). The study is limited to the range of ages 5x10^6^ to 8x10^8^yr, which is free from contamination by Horizontal Branch stars. It is shown that in this range of ages the theoretical sequence C(15-31) vs. age agrees well with the one derived by combining the observed colour index C(15-31) with the ages determined via isochrone fitting to the colour-magnitude diagrams while systematic differences, which are discussed on here, exist with respect to the age calibration by Meurer, Cacciari and Freeman (1990). The present age calibration C(15-31) vs. log(t), provided in an analytical form, is finally used to determine the ages of the 29 clusters in our sample, including 13 objects for which no determination was available via the isochrone fitting method.