Far Ultraviolet Photometry of Globular Clusters
Session 116 -- Globular Clusters and Compact Objects
Oral presentation, Thursday, 12, 1995, 10:00am - 11:30am

[116.01D] Far Ultraviolet Photometry of Globular Clusters

J. H. Whitney, R.W. O'Connell, R.T. Rood (UVa)

Far-Ultraviolet exposures (centroid 1620 \AA; bandwidth 225 \AA; field of view 40 arcmin) were obtained of the globular clusters $\omega$ Cen (NGC 5139), M3 (NGC 5272), and M13 (NGC 6205) with the Ultraviolet Imaging Telescope (UIT) during the Astro-1 mission of December 1990. The 291 sec exposure of $\omega$ Cen extends to a far-ultraviolet (FUV) magnitude of 16.4, providing a complete sample of 1957 hot horizontal branch (HB) and supra-HB stars even into the dense core of the cluster. The far-UV observations of $\omega$ Cen were combined with Str\"omgren $u$ CCD observations from CTIO to produce a FUV, $u$ color-magnitude diagram. The CMD permits analysis of the HB structure, showing a large population of supra-HB stars, a thickly populated extreme HB, and a discontinuity at T$_{e} = 16000\,$K. The 199 sec exposure of M3 extends to a FUV magnitude of 16.2, sufficient to detect the hottest HB stars and all the hot supra-HB stars. Over 70 sources are detected. The 46 sec exposure of M13 provides photometry for over 40 stars, most of them supra-HB objects, as the FUV magnitude limit is approximately 14.2, slightly above the expected FUV magnitude of the HB. Compared to the most comprehensive grid of HB and post-HB evolutionary models (Dorman et al. 1993, ApJ, 419, 596), the observed numbers of supra-HB stars appear consistent with the evolution of hot HB stars into supra-HB stars. The clusters with bluer HB morphology ($\omega$ Cen, M13) produce proportionally more supra-HB stars than the cluster without the blue morphology (M3).