An extended main sequence turn-off in the Small Magellanic Cloud star
  cluster NGC411
  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 NGC411. This is the second case of
an eMSTO being identified in a star cluster belonging to the Small Magellanic
Cloud (SMC), after NGC419. Despite the present masses of these two SMC clusters
differ by a factor of 4, the comparison between their colour--magnitude
diagrams (CMD) 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. NGC411, however, presents merely a trace of secondary
red clump as opposed to its prominent manifestation in NGC419. This could be
due either to the small number statistics in NGC411, or by the star formation
in NGC419 having continued for 60 Myr longer than in NGC411. 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 NGC419 than in NGC411. We discuss these findings in the context of
possible scenarios for the origin of eMSTOs.
