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

We present far-ultraviolet images of the globular cluster omega Centau ri obtained with the Ultraviolet Imaging Telescope (UIT) during the 19 90 December Astro-1 mission. A total of 1957 sources are detected at 1 620 Angstrom to a limiting UV magnitude of 16.4 in the central 24' dia meter region of the field and a limit of 15.6 over the remainder of th e 40' diameter field. Over 1400 of these sources are matched with star s on a Stromgren u band CCD frame obtained with the CTIO 0.9 m telesco pe to produce a (far-UV, u) color-magnitude diagram. Completeness of t he sample and error estimates are determined by photometry of artifici al 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 si milar to 16 000 K. There is also a significant population of stars abo ve 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 Asy mptotic Giant Branch'' stars. We compare the observations to recent th eoretical evolutionary tracks for the zero-age HB and subsequent phase s. 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 g reater than or similar to 50 000 K, which are hotter than zero-age HB stars with envelope masses of 0.003 M(circle dot) .