ULTRAVIOLET IMAGERY OF NGC-6752 - A TEST OF EXTREME HORIZONTAL-BRANCHMODELS

We present a 1620 Angstrom image of the nearby globular cluster NGC 67 52 obtained with the Ultraviolet Imaging Telescope (UIT) during the As tro-2 mission of the Space Shuttle Endeavour in 1995 March. An ultravi olet-visible color-magnitude diagram (CMD) is derived for 216 stars ma tched with the visible photometry of Buonanno et al. This CMD provides a nearly complete census of the hot horizontal-branch (HB) population with good temperature and luminosity discrimination for comparison wi th theoretical tracks. The observed data show good agreement with the theoretical zero-age horizontal branch (ZAHB) of Sweigart for an assum ed reddening of E(B-V) = 0.05 and a distance modulus of 13.05. The obs erved HB luminosity width is in excellent agreement with the theoretic al models and supports the single-star scenario for the origin of extr eme horizontal branch (EHB) stars. However, only four stars can be ide ntified as post-EHB stars, whereas almost three times this many are ex pected from the HB number counts. If this effect is not a statistical anomaly, then some noncanonical effect may be decreasing the post-EHB lifetime. The recent noncanonical models of Sweigart, which have heliu m-enriched envelopes due to mixing along the red giant branch, cannot explain the deficit of post-EHB stars, but might be better able to exp lain their luminosity distribution.