We use the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) to obtain deep, high resolution images of two intermediate-age star clusters in the Large Magellanic Cloud of relatively low mass ($approx$ $10^4$ $M_{odot}$) and signific
antly different core radii, namely NGC2209 and NGC2249. For comparison purposes, we also re-analyzed archival HST images of NGC1795 and IC2146, two other relatively low mass star clusters. From the comparison of the observed color-magnitude diagrams with Monte Carlo simulations, we find that the main sequence turnoff (MSTO) regions in NGC2209 and NGC2249 are significantly wider than that derived from simulations of simple stellar populations, while those in NGC1795 and IC2146 are not. We determine the evolution of the clusters masses and escape velocities from an age of 10 Myr to the present age. We find that the differences among these clusters can be explained by dynamical evolution arguments if the currently extended clusters (NGC2209 and IC2146) experienced stronger levels of initial mass segregation than the currently compact ones (NGC2249 and NGC1795). Under this assumption, we find that NGC2209 and NGC2249 have estimated escape velocities $V_{rm esc}$ $geq$ 15 km s$^{-1}$ at an age of 10 Myr, large enough to retain material ejected by slow winds of first-generation stars, while the two clusters that do not feature extended MSTOs have $V_{rm esc}$ $leq$ 12 km s$^{-1}$ at that age. These results suggest that the extended MSTO phenomenon can be better explained by a range of stellar ages rather than a range of stellar rotation velocities or interacting binaries.
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 id
entified 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.
Open and globular star clusters have served as benchmarks for the study of stellar evolution due to their supposed nature as simple stellar populations of the same age and metallicity. After a brief review of some of the pioneering work that establis
hed the importance of imaging stars in these systems, we focus on several recent studies that have challenged our fundamental picture of star clusters. These new studies indicate that star clusters can very well harbour multiple stellar populations, possibly formed through self-enrichment processes from the first-generation stars that evolved through post-main-sequence evolutionary phases. Correctly interpreting stellar evolution in such systems is tied to our understanding of both chemical-enrichment mechanisms, including stellar mass loss along the giant branches, and the dynamical state of the cluster. We illustrate recent imaging, spectroscopic and theoretical studies that have begun to shed new light on the evolutionary processes that occur within star clusters.
