AAS 195th Meeting, January 2000
Session 127. Variable Stars
Oral, Saturday, January 15, 2000, 2:00-3:30pm, Centennial III

## [127.02] Rotation Velocities and Photospheric Abundances of Blue Horizontal-Branch Stars in Globular Clusters

B. B. Behr (Department of Astronomy, California Institute of Technology)

High-resolution spectroscopy of blue horizontal-branch (BHB) stars in several metal-poor globular clusters reveals that stars bluewards of the photometric `gap' at B-V ~0 have significantly different photospheric compositions and v \sin i rotation rates than cooler stars. Iron and other metals are enhanced by factors of 30 to 1000 in the hotter stars, while helium is underabundant by as much as 2 orders of magnitude. These abundance anomalies are most likely due to diffusion effects --- gravitational settling of helium, and radiative levitation of the metals --- in the stellar atmospheres, and the resulting change in the emitted spectral energy distribution may give rise to the observed photometric features of the cluster CMDs. The hotter stars also exhibit modest rotations, v \sin i < 10~km~s{}-1, in sharp contrast to v \sin i of 40~km~s{}-1 measured for some of the cooler stars. This slow rotation may be a prerequisite for diffusion to take place, and also impacts theories of RGB mass loss.

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