Two groups of nearly coeval star clusters in the Small Magellanic Cloud

We report new photometry and main-sequence turnoff ages for seven populous star clusters in the SMC with M-v < -6 and age greater than 1 Gyr, using th e Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In con trast to the accepted picture, these clusters appear to have formed in two brief intervals, the oldest 8 +/- 2 Gyr ago and one during a more recent bu rst 2 +/- 0.5 Gyr ago. When the ridgelines of the four clusters (NGC 339, 3 61, and 416 and Kron 3) in the 8 Gyr burst are aligned, the dispersion in t urnoff luminosities is less than 0.2 mag, corresponding to a maximum age sp read of +/-0.7 Gyr. When the ridgelines of three clusters (NGC 152, 411, an d 419) in the 2 Gyr burst are aligned, the maximum dispersion of 0.2 mag in turnoff luminosity corresponds to a permitted age spread of +/-0.2 Gyr. Wi thin each group of clusters, the entire cluster loci (including red giant b ranches and clumps) are nearly identical, consistent with a very small spre ad in metallicity and age. In contrast to the wide dispersion in ages previ ously reported in the literature, our sample with more precise photometry a nd age measurements supports a burst-punctuated rather than a continuous cl uster formation history for the oldest SMC clusters.