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Session 86 - Measurement of Cosmological Parameters.
Oral session, Wednesday, January 17
1st Floor, La Villita Assembly Building
The UIT on Astro-1 found nearly 2000 hot stars in the globular cluster Omega Centauri (Whitney et al.\ 1994, AJ, 108, 1350), of which 28 are at least 1 mag brighter than the horizontal branch (HB). We have obtained CTIO 4m and/or IUE low-dispersion spectra of eleven of the brightest of these stars. All seven stars observed at CTIO are radial velocity members of the clusters. Three of these stars have log Teff > 60,000 K and show only Balmer lines and He II lines. These stars, which appear to be in the post-asymptotic giant branch phase, are the hottest stars ever found in a globular cluster. The other four stars have log Teff \sim 20,000 K and show a B-type spectrum with numerous He I lines but none of He II. We have estimated the helium abundance by comparison with hydrogen-helium NLTE model stellar atmospheres. The He II lines in the hottest three stars in our sample appear to be best fit by models with a He/H ratio of about 0.1. This helium abundance should reflect the primordial helium abundance of the gas from which Omega Cen formed nearly 13 \times 10^9 yr ago, except for the modest helium enrichment caused by the first dredge-up during the red giant branch phase. By analyzing these stars we have been able to derive a helium abundance for a globular cluster which is independent of the well-known R-method and which should provide an estimate of the helium abundance produced by the Big Bang. The four cooler stars in our sample show a quite different pattern of helium abundances. The three faintest of these stars have a substantially lower helium abundance which we attribute to the effects of diffusion during the preceding HB phase. These stars appear to have recently evolved off of the blue end of the HB. In contrast, the most luminous of these stars is very helium rich (He/H = 0.55), indicating that some UV-bright stars undergo interior mixing during their preceding AGB evolution.
