Observations of the Ultraviolet-Bright Star in 47 Tucanae with the Hopkins Ultraviolet Telescope

Abstract

The ultraviolet-bright star 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 spectrum extends from 912 to 1840 A with a resolution of about 3 A. It has numerous absorption features, but no emission features other than terrestrial airglow lines. We have fitted the data with Kurucz stellar atmosphere models using a χ2 minimization technique. The Kurucz model that best fits the observed spectrum has Teff = 11,000 K, log g = 2.5, and [M/H] = -1.0; however, it overpredicts the flux in the sub-Lyβ region by nearly a factor of 3. Kurucz line profiles fitted to the star's Hδ 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 (AGB) evolutionary tracks. We measure the equivalent widths of the star's absorption lines and find them consistent with the abundances of cluster giant-branch and AGB stars. We conclude that BS in 47 Tuc is a cluster member and that non-LTE effects and nonsolar abundance ratios not included in the Kurucz models are important in this star.