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STScI Preprint #1266
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute. ST ScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555
We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Space Telescope and WFPC2. Our photometry reaches 1.5 mag below the main sequence turnoff, with negligible field contamination. A blue straggler star (BSS) sequence of 42 candidates is clearly seen, the first such detection in an extragalactic globular cluster. The BSS are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and completeness tests demonstrate that at least 23 of our candidates are genuine BSS. The BSS extend about 1.8 magnitudes brighter than the main sequence turnoff, in accord with similar sequences in Galactic Globular clusters. One BSS candidate is 2.4 magnitudes brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearly detected and well populated, with a modest blueward extension. The relaxation times of the cluster stars and the BSS are such that most BSS are in energy equipartition with the other stars. We find VHBTO=3.3± 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that of most Galactic globular clusters, and of one of the oldest LMC globulars.
1) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
2) Department of Astronomy Columbia University New York, NY, 10027