Ages, Distances, And Fiducials For Seven Smc Star Clusters With Hst/acs

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As the closest star forming dwarf galaxy, the Small Magellanic Cloud (SMC) is the preferred location for detailed studies of so-called populous star clusters, dense stellar systems with no immediate counterparts in the Milky Way. Moreover, in contrast to the Milky Way and most other galaxies in the Local Group, the SMC retains massive, compact star clusters that formed continuously throughout the last 11 Gyr; even during the period of quiescence observed in the Large Magellanic Cloud (LMC). We observed the star clusters NGC121, Lindsay1, Kron3, NGC339, NGC416, Lindsay38, and NGC419 with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Our data yield color-magnitude diagrams in the F555W and F814W filters that reach at least 3 mag below the main-sequence turnoffs. We derive the ages of the clusters using the scaled solar isochrone models of the Padova, Teramo, and Dartmouth groups. For NGC121, the oldest and only ''true'' globular cluster in the SMC, the best isochrone fit was obtained with an
α-enhanced ([Fe/α]=0.2) Dartmouth isochrone, which gives
an age of 10.5 ± 0.5 Gyr. We confirm that NGC121 is 2--3 Gyr
younger than the oldest Galactic globular clusters of comparable metallicity as well as its oldest counterparts in the LMC, Fornax, and Sagittarius. The remaining six clusters in our sample have ages between 6 and 8.3 Gyr. Our deep HST photometry is complemented by ground-based spectroscopy obtained with the Very Large Telescope at the European Southern Observatory. Combining our photometric ages and spectroscopic metallicities, we find that SMC clusters of similar age may differ by several tenths of dex in metallicity. Hence the SMC cannot have experienced a simple chemical evolution where abundances uniformly rise over time.

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