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Published November 1, 1994 | Published
Journal Article Open

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


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.

Additional Information

© 1994 American Astronomical Society. Provided by the NASA Astrophysics Data System. Received 1994 May I6; accepted 1994 August 11. The success of WFPC2 has been made possible by the dedicated efforts of many people, including the project engineering staff at JPL and the flight and support crew of STS-61. We are grateful to Ben Dorman for producing synthetic magnitudes at short notice, to Bob Rood and again Ben Dorman for valuable discussions and comments on earlier versions of the manuscript, to Haldan Cohn for the U surface brightness profile, to Mark Clampin for the spectrum of GRW + 70°5824, to Ron Gilliland for a copy of his paper prior to submission, and to an anonymous referee for several useful suggestions. This research has made use of the Simbad database, operated by CDS, Strasbourg, France, and was carried out by the WFPC2 Science Team for JPL with the support of NASA contract NAS 7-1260.

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Published - 1994ApJ___435L__55W.pdf


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August 20, 2023
August 20, 2023