Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of 18 blue horizontal-branch stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at T_eff~8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the vsini rotations appear to have a bimodal distribution, with eight stars at vsini<15 km s^-1 and two stars at vsini~35 km s^-1. Most of the stars at T_eff>=10,000 K, however, are slowly rotating, vsini<7 km s^-1, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of T_eff, and helium is depleted by factors of 10-30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars. Based in large part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California.