Hubble Space Telescope observations of NGC 121: First detection of blue stragglers in an extragalactic globular cluster

We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Spade Telescope and Wide Field Plan etary Camera 2. Our photometry reaches 1.5 mag below the main-sequence turn off, with negligible field contamination. A blue straggler star (BSS) seque nce of 42 candidates is clearly seen-the first such detection in an extraga lactic globular cluster. The BSSs are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and comp leteness tests demonstrate that at least 23 of our candidates are genuine B SSs. The BSSs extend about 1.8 mag brighter than the main-sequence turnoff, in accord with similar sequences in Galactic globular clusters. One BSS ca ndidate is 2.4 mag brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearl y detected and well populated, with a modest blueward extension. The relaxa tion times of the cluster stars and the BSSs are such that most BSSs are in energy equipartition with the other stars. We find Delta V-TO(HB) = 3.3 +/ - 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that o f most Galactic globular clusters and of one of the oldest LMC globulars.