OBSERVATIONS OF THE ULTRAVIOLET-BRIGHT STAR IN 47-TUCANAE WITH THE HOPKINS-ULTRAVIOLET-TELESCOPE

The ultraviolet-bright star ''BS'' in the globular cluster 47 Tuc (NGC 104) was observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 Space Shuttle mission in 1995 March. The resulting spectr um extends from 912 to 1840 Angstrom with a resolution of about 3 Angs trom. It has numerous absorption features, but no emission features ot her than terrestrial airglow lines. We have fitted the data with Kuruc z stellar atmosphere models using a chi(2) minimization technique. The Kurucz model that best fits the observed spectrum has T-eff = 11,000 K, log g = 2.5, and [M/H] = -1.0; however, it overpredicts the flux in the sub-Ly beta region by nearly a factor of 3. Kurucz line profiles fitted to the star's H delta Balmer line yield log g = 2.0. Assuming a distance of 4.6 +/- 1.0 kpc, we estimate the stellar luminosity to be log L/L. = 3.07 +/- 0.20. These atmospheric parameters place the star on the Schonberner post-asymptotic giant branch (AGE) evolutionary tr acks. We measure the equivalent widths of the star's absorption lines and find them consistent with the abundances of cluster giant-branch a nd AGE stars. We conclude that BS in 47 Tuc is a cluster member and th at non-LTE effects and nonsolar abundance ratios not included in the K urucz models are important in this star.