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Paper: Primordial Lithium: Keck Observations in M 92 Turn-Off Stars
Volume: 154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop
Page: 913
Authors: Boesgaard, Ann Merchant; Deliyannis, Constantine P.; Stephens, Alex; King, Jeremy R.
Abstract: We present new Keck I/HIRES observations at R=45,000 (= 3 pixels) of seven stars near the turnoff of the old, metal-poor globular cluster M 92. In three of these stars we have S/N ratios of 40 per pixel and in the other four the S/N is near 20. We find that star 18 has a high Li abundance, log N(Li) = 2.5, which is about a factor of three larger than that in stars 21 and 46. The Li abundance in star 18 is high compared to the halo field star plateau, and is similar to that in the remarkable Li-rich halo field star BD+23 deg3912. In addition to the high Li abundance in star 18, there is a dispersion in Li abundance in our 7 stars covering the full range of a factor of three. We have attempted to determine whether the high Li in star 18 is due to less-than-average Li depletion in this star from an even higher initial abundance, as predicted by the Yale rotational models, or is due to the extraordinary action of Li production mechanisms in the material that formed this star. We have found no convincing evidence that favors Li production: 1) Stars 18, 21, and 46 have identical Ba abundances, which argues against Li production carrying an s-process signature. 2) These three stars have indistinguishable Ca, Cr, Fe, and Ti, which argues against supernova Li production. 3) We discuss nu-process production of Li and find no convincing observational evidence for this from the strengths of the Mg, Ca, and Fe lines. 4) The similarity in age of these cluster stars argues against cosmic ray Li production that requires age differences of Gyr. The most likely explanation for the Li dispersion is differential Li depletion from a (possibly significantly) higher primordial Li abundance due to differences in the initial angular momentum in each star followed by spin-down; the most rapid rotators destroy the most Li, whereas the initially slower rotators preserve more Li.
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