EVIDENCE FOR A DISPERSION IN THE LITHIUM ABUNDANCES OF EXTREME HALO STARS

Knowledge of the primordial lithium abundance (Li(p)) helps to constra in cosmological parameters and to discriminate between models of primo rdial nucleosynthesis; the best current probe of Li, is the set of lit hium abundances measured in old halo stars, provided that stellar Li d epletion mechanisms are taken into account. Standard stellar evolution ary models show little depletion (Deliyannis, Demarque, & Kawaler), co nsistent with standard big bang nucleosynthesis (BBN), whereas models with rotational mixing deplete about an order of magnitude (Pinsonneau lt, Deliyannis, & Demarque), possibly consistent with inhomogeneous BB N, or other nonstandard scenarios. One way to test the rotational mode ls is through their prediction that a small dispersion can result in t he lithium abundances if there is a significant dispersion in the init ial angular momenta. For all available extreme halo dwarf data, we hav e performed a detailed analysis in the (b - y) color-lithium equivalen t width plane, which eliminates many of the uncertainties associated w ith the transformation to the T(eff)-abundance plane. Furthermore, we have used two independent means to compute the uncertainties: one that incorporates detailed information about the individual S/N and releva nt instrumentation, and one that utilizes empirically derived uncertai nties from multiple observations; our conclusion is the same in either case. Using chi2 tests, we find evidence for a small dispersion (of o rder +/-20% about the mean) in the Li equivalent width at fixed color, with a high degree of confidence. Interpreted according to usual stan dard model atmospheres theory, this translates directly into a small d ispersion in the Li abundances themselves. This dispersion is consiste nt with, but does not uniquely support, the rotational depletion. We h ave also corrected the colors for reddening, and the evidence for disp ersion remains strong. One star, BD 23-degrees-3912, lies more than 50 % above the lithium plateau.