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Published March 1, 2000 | Published + Accepted Version
Journal Article Open

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 v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km s^(-1) and two stars at v sin i ~ 35 km s^(-1). Most of the stars at T_(eff) ≥ 10,000 K, however, are slowly rotating, v sin i < 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.

Additional Information

© 2000 The American Astronomical Society. Received 1999 November 25; accepted 2000 January 12; published 2000 February 9. 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. These observations would not have been feasible without the HIRES spectrograph and the Keck I telescope. We are indebted to Jerry Nelson, Gerry Smith, Steve Vogt, and many others for making such marvelous machines, to the W. M. Keck Foundation for making it happen, and to a bevy of Keck observing assistants for making them work. J. G. C. and B. B. B. are grateful for support from NSF grant AST 98-19614.

Attached Files

Published - Behr_2000_ApJ_531_L37.pdf

Accepted Version - 0002119.pdf


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