2013MNRAS.431.3501G

Based on new observations with the Wide Field Camera 3 onboard the Hubble Space Telescope, we report the discovery of an extended main-sequence turn-off (eMSTO) in the intermediate-age star cluster NGC 411. This is the second case of an eMSTO being identified in a star cluster belonging to the Small Magellanic Cloud (SMC), after NGC 419. Despite that the present masses of these two SMC clusters differ by a factor of ∼4, the comparison between their colour-magnitude diagrams (CMDs) shows striking similarities, especially regarding the shape of their eMSTOs. The loci of main CMD features are so similar that they can be well described, in a first approximation, by the same mean metallicity, distance and extinction. NGC 411, however, presents merely a trace of secondary red clump as opposed to its prominent manifestation in NGC 419. This could be either due to the small number statistics in NGC 411 or due to the star formation in NGC 419 having continued for ∼60 Myr longer than in NGC 411. Under the assumption that the eMSTOs are caused by different generations of stars at increasing age, both clusters are nearly coeval in their first episodes of star formation. The initial period of star formation, however, is slightly more marked in NGC 419 than in NGC 411. We discuss these findings in the context of possible scenarios for the origin of eMSTOs.