2010MNRAS.403.1156R

The rich Small Magellanic Cloud (SMC) star cluster NGC 419 has recently been found to present both a broad main-sequence turn-off and a dual red clump of giants in the sharp colour-magnitude diagrams (CMDs) derived from the High-Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope. In this work, we apply to the NGC 419 data the classical method of star-formation-history (SFH) recovery through CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star-formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak in the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star-formation episodes (or continuous star formation) being at the origin of the multiple main-sequence turn-offs in Magellanic Cloud clusters with ages of about 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main-sequence spread caused by rotation, could produce similarly good fits to the data.