Halo star lithium depletion

The depletion of lithium during the pre-main-sequence and main-sequence pha ses of stellar evolution plays a crucial role in the comparison of the pred ictions of big bang nucleosynthesis with the abundances observed in halo st ars. Previous work has indicated a wide range of possible depletion factors , ranging from minimal in standard (nonrotating) stellar models to as much as an order of magnitude in models that include rotational mixing. Recent p rogress in the study of the angular momentum evolution of low-mass stars pe rmits the construction of theoretical models capable of reproducing the ang ular momentum evolution of low-mass open cluster stars. The distribution of initial angular momenta can be inferred from stellar rotation data in youn g open clusters. In this paper we report on the application of these models to the study of lithium depletion in main-sequence halo stars. A range of initial angular momenta produces a range of lithium depletion factors on th e main sequence. Using the distribution of initial conditions inferred from young open clusters leads to a well-defined halo lithium plateau with mode st scatter and a small population of outliers. The mass-dependent angular m omentum loss law inferred from open cluster studies produces a nearly flat plateau, unlike previous models that exhibited a downward curvature for hot ter temperatures in the Li-7-T-eff plane. The overall depletion factor for the plateau stars is sensitive primarily to the solar initial angular momen tum used in the calibration for the mixing diffusion coefficients. Uncertai nties remain in the treatment of the internal angular momentum transport in the models, and the potential impact of these uncertainties on our results is discussed. The Li-6/Li-7 depletion ratio is also examined. We find that the dispersion in the plateau and the Li-6/Li-7 depletion ratio scale with the absolute Li-7 depletion in the plateau, and we use observational data to set bounds on the Li-7 depletion in main-sequence halo stars. A maximum of 0.4 dex depletion is set by the observed dispersion and Li-6/Li-7 deplet ion ratio, and a minimum of 0.2 dex depletion is required by both the prese nce of highly overdepleted halo stars and consistency with the solar and op en cluster Li-7 data. The cosmological implications of these bounds on the primordial abundance of Li-7 are discussed.