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Session 86 - Rotating, Modeling and Blue Stars.
Oral session, Wednesday, January 15
Harbour C,

[86.04] Extreme and Intermediate Blue Horizontal Branch Stars

N. D'Cruz, R. Rood, R. O'Connell (UVa), B. Dorman (UVa, NASA/GSFC), R. Dickens (Bristol)

The blue horizontal branch population consists of extreme horizontal branch (EHB) stars and the cooler intermediate blue horizontal branch (IBHB) stars. EHB stars have very tiny hydrogen envelopes (\leq 0.05\,M_ødot) as a result of extreme mass loss during the red giant branch (RGB) phase. They occupy a small range in mass and evolve at high effective temperatures after core He exhaustion. Their prodigious UV emission makes them the leading contenders for the source of the ultraviolet excess phenomenon seen in elliptical galaxies and spiral bulges.

Using our stellar evolution code, STEV, we have studied the effects of extreme RGB mass loss on the HB over the metallicity range -2.26 \leq [Fe/H] \leq +0.37. Mass loss followed Reimers' mass loss formula. Extreme mass loss causes some stars to end their lives as He white dwarfs; the others ignite He at high temperatures and form a ``blue hook'' at the blue end of the canonical Zero-Age HB. We considered the distribution of initial HB masses as a probability distribution in Reimers' mass loss efficiency parameter, \eta_\scriptscriptstyle R, rather than directly as a mass probability distribution. For globular cluster abundances, the range of \eta_\scriptscriptstyle R producing EHB stars was comparable to that producing normal HB stars. As metallicity is increased, the range and magnitude of \eta_\scriptscriptstyle R varies only slightly, whereas the range of \eta_\scriptscriptstyle R producing mid-HB stars becomes very small, implying that the HB will be bimodal provided \eta_\scriptscriptstyle R is large enough. Further details can be found in ApJ, 466, 359 (1996).

To determine how \eta_\scriptscriptstyle R might vary with metallicty, we have obtained fiber spectra of IBHB stars in the globular cluster ømega Cen using the 3.9\,m Anglo-Australian telescope. T_eff, \log g and [Ca/H] are being determined from the spectra. Our initial analysis indicates that for stars in the 7500 - 8200\, K range, the distribution of [Ca/H] in the IBHB stars is different from that of ømega Cen's giants and variables. There are comparatively more lower metallicity IBHB stars. This work is still in progress.

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