AN ULTRAVIOLET-VISIBLE INVESTIGATION OF THE GLOBULAR-CLUSTER NGC-1851

Two-color ultraviolet images of the globular cluster NGC 1851 were obt ained with the Ultraviolet Imaging Telescope (UIT) during the 1990 Dec ember Astro-1 Spacelab mission. A total of 133 stars are detected at 2 490 angstrom and 74 stars at 1520 angstrom. An ultraviolet color-magni tude diagram based on the 46 well-photometered stars that appear in bo th images is presented. Thirty-nine of the 45 horizontal branch (HB) s tars fall below the zero-age horizontal branch (ZAHB) of Z = 0.001 and Y = 0.23 predicted by Sweigart by as much as 0.6 mag if the interstel lar reddening to the cluster is E(B - V) = 0.02. Supporting ground-bas ed V and B observations, however, show excellent correlation with the same model ZAHB. A newly detected hot subdwarf star with T(eff) approx imately 26,000 K appears to be an extreme horizontal branch star. The measured flux for this star also falls approximately 0.6 mag below the position on the CMD predicted by parameters derived from ground-based spectroscopy by Landsman. The far-ultraviolet image is dominated by t he ultraviolet-bright member star UV 5, which contributes 30% of the t otal flux at 1520 angstrom. The UIT photometry is consistent with the classification of UV 5 as a post-asymptotic giant branch (PAGB) star. To a limit of 16.5 mag at 1520 angstrom we find no ultraviolet counter part within 6'' of the position of the X-ray source MX 0513 - 40.