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Session 28 - Stellar Evolution: Beyond the Main Sequence.
Oral session, Monday, June 10
Humanities 3650,
The nearby globular cluster NGC 6752 was observed with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Space Shuttle mission in 1995 March. We detect 348 hot horizontal branch (HB) star candidates on the image. This number should be nearly a complete census of the HB population because the UIT field of view of 40' is well matched to the cluster diameter, (2) the solar-blind detectors on UIT suppress the cool star population, and minimize crowding effects even in the cluster core, and (3) the ground-based color-magnitude diagram (CMD) of Buonanno et al.\ (1986) shows that the HB contains only hot stars.
We present an ultraviolet -- visible CMD for the 211 stars with ground-based photometry available from the Buonanno et al.\ catalog. The UIT absolute calibration is well-determined from comparison with 18 hot HB stars in the cluster observed with IUE. Four hot (Teff \sim 35,000 K) luminous (\log L/Lsun = 2.1) stars in the CMD are plausibly identified as being directly descended off of the extreme horizontal branch (EHB). However, the large luminosity separation between these post-EHB stars and the EHB is not predicted by canonical models of HB and post-HB evolution. We show that this luminosity separation and the numbers of post-EHB stars can be explained by noncanonical models whose envelopes have been enriched in helium by internal mixing during the preceding red-giant-branch phase.
