Strong mass loss on the red giant branch (RGB) can result in the formation of extreme horizontal branch (EHB) stars. The EHB stars spend most of their He core and shell burning phase at high temperatures and produce copious ultraviolet flux. They have very small hydrogen envelopes and occupy a small range in mass. We have computed evolutionary RGB models with mass loss for stars with a range of metallicities at initial masses < 1.1 Msun corresponding to populations ages between 12.5 and 14.5 Gyr. We used the Reimers formula to characterize mass loss, but investigated a larger range of the mass loss efficiency parameter, eta, than is common. To understand how the number of EHB stars varies with metallicity in a stellar population we considered how the zero-age horizontal branch (ZAHB) is populated. The range in eta producing EHB stars is comparable to that producing `mid-HB' stars. Somewhat surprisingly, neither the range nor magnitude of eta producing EHB stars varies much metallicity. In contrast, the range of eta producing mid-HB stars decreases with increasing metallicity. Hence the HB of populations with solar metallicity and higher, such as expected in elliptical galaxies and spiral bulges, will be bimodal if the distribution covers a sufficiently large range in eta.Comment: AASLaTeX v.4, 29pp., postscript available at http://shemesh.gsfc.nasa.gov/~dorman/Ben.htm

The globular cluster u Centauri contains the largest known population of very hot horizontal-branch (HB) stars. We have used the Hubble Space T elescope to obtain a far-UV/optical color-magnitude diagram of three Ðelds in u Cen. We Ðnd that over 30% of the HB objects are "" extreme ÏÏ HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a signiÐcant population of hot sub-HB stars, which we interpret as the "" blue-hook ÏÏ objects previously predicted by DÏCruz et al. These are produced by late He Ñashes in stars which have undergone unusually large giant branch mass loss. The cluster u Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass-loss efficiency which increases with metallicity.

We analyze the far-UV and Stro mgren u photometric data of the globular cluster u Cen presented in an earlier paper. The color-magnitude diagram of the cluster from these two bands shows that u CenÏs horizontal-branch (HB) consists of a group of cooler intermediate blue HB (IBHB) and a group of extreme HB (EHB) stars, together with a large population of post-HB stars. Unexpected features in the diagram are a discontinuity between the EHB and IBHB objects lying at an, unusually large population of stars below the EHB, and a number of sources bluer than an inÐnite temperature blackbody.No adjustment of the assumed reddening or distance modulus parameters satisfactorily explains either the sub-HB or very hot star components observed. The radial distributions of the IBHB and EHB subpopulations are similar after corrections for completeness and crowding are made. This result, as well as the fact that u CenÏs core is not dynamically evolved, implies that dynamical e †ects are not required for the production of EHB stars in globular clusters.To compare the observations to theory we use Hess diagrams, which describe the density distribution of stars in the color-magnitude diagram. To simulate the known abundance spread in u Cen, we use image-processing "" morphing ÏÏ techniques to create a composite Hess diagram for [Fe/H] \ [2.2, [1.5, and [0.5. We populate the zero-age HB (ZAHB) in our simulations assuming either Ñat or Gaussian distributions in total stellar mass or, alternatively, distributions in the red giant branch mass-loss efficiency parameter g in ReimersÏs formula. Our ZAHBÏs extend to the lowest possible ZAHB mass as determined by evolving models along the red giant branch with extreme mass-loss rates. Neither Ñat nor Gaussian distributions in ZAHB mass reproduce the observed HB gap or the sub-HB population. However, the g distribution models can crudely reproduce the gap as well as the sub-HB population while simultaneously Ðtting the rest of the HB and post-HB population.

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