Far-ultraviolet imaging of the globular cluster NGC 6681 with WFPC2

Alan M. Watson, Jeremy R. Mould, John S. Gallagher, Gilda E. Ballester, Christopher J. Burrows, Stefano Casertano, John T. Clarke, David Crisp, Richard E. Griffiths, J. Jeff Hester, John G. Hoessel, Jon A. Holtzman, Paul Scowen, Karl R. Stapelfeldt, John T. Trauger, James A. Westphal

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


We have imaged the globular cluster NGC 6681 in the far-UV and visible with WFPC2 on the 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.

Original languageEnglish (US)
Pages (from-to)L55-L58
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - Nov 1 1994


  • Globular clusters: individual: (NGC 6681)
  • Stars: evolution
  • Stars: horizontal-branch
  • Ultraviolet: stars

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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