Far-Ultraviolet Imaging of the Globular Cluster NGC 6681 with WFPC2
Casertano, Stefano; Ballester, Gilda E.; Crisp, David; Holtzman, Jon A.; Trauger, John T.; Gallagher, John S., III; Hester, J. Jeff; Westphal, James A.; Mould, Jeremy R.; Hoessel, John G.; Clarke, John T.; Stapelfeldt, Karl R.; Burrows, Christopher J.; Griffiths, Richard E.; Scowen, Paul A.; Watson, Alan M.
United States, Australia
Abstract
We have imagined the globular cluster NGC 6681 in the far-UV and visible with Wide Field/Planetary Camera-2 (WFPC2) on the Hubble Space Telescope (HST). Our far-UV images show a sparsely populated and fully resolved central region, and we detect 122 stars. The far-UV to visible color-magnitude diagram shows a well-defined horizontal branch with no evidence for hot, more evolved descendants. We find one hot horizontal-branch star significantly below the model zero-age horizontal branch, but the rest are consistent with evolutionary models within uncertainties in calibration, distance, and reddening. The center of the cluster harbors two luminous blue stragglers. Our far-UV images graphically confirm that there is no steep density gradient at small radii among the horizontal-branch stars of this post-core-collapse cluster and show no evidence for significant color gradients.