FAR-ULTRAVIOLET PHOTOMETRY OF THE GLOBULAR-CLUSTER OMEGA-CEN

We present far-ultraviolet images of the globular cluster omega Centauri obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 mission. A total of 1957 sources are detected at 1620 Angstrom to a limiting UV magnitude of 16.4 in the central 24' diameter region of the field and a limit of 15.6 over the remainder of the 40' diameter field. Over 1400 of these sources are matched with stars on a Stromgren u band CCD frame obtained with the CTIO 0.9 m telescope to produce a (far-UV, u) color-magnitude diagram. Completeness of the sample and error estimates are determined by photometry of artificial stars added to the images. The horizontal branch (HB) of the CMD is heavily populated hotter than 9000 It. A large number of ''extreme HB'' stars are found hotter than a conspicuous break in the HB at T-e similar to 16 000 K. There is also a significant population of stars above the HB, the brightest of which is 4 mag brighter than the HB. Most of the hotter of these appear to be ''AGB-manque'' or ''Post-Early Asymptotic Giant Branch'' stars. We compare the observations to recent theoretical evolutionary tracks for the zero-age HB and subsequent phases. The tracks match the data well, with the exception of the hotter HB stars, many of which fall below the zero-age horizontal branch. It is unclear as yet whether these are a special population or an artifact of errors in the models or photometry. We identify 33 stars with T-e greater than or similar to 50 000 K, which are hotter than zero-age HB stars with envelope masses of 0.003 M(circle dot) .