Halo Star Lithium Depletion

The depletion of lithium during the pre-main sequence and main sequence phases of stellar evolution plays a crucial role in the comparison of the predictions of big bang nucleosynthesis with the abundances observed in halo stars. Previous work has indicated a wide range of possible depletion factors, ranging from minimal in standard (non-rotating) stellar models to as much as an order of magnitude in models which include rotational mixing. Recent progress in the study of the angular momentum evolution of low mass stars (Krishnamurthi et al. 1997a) permits the construction of theoretical models which reproduce the angular momentum evolution of low mass open cluster stars. The distribution of initial angular momenta can be inferred from stellar rotation data in young 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 the main sequence. Using the distribution of initial conditions inferred from young open clusters leads to a well-defined halo lithium plateau with modest scatter and a small population of outliers. The mass dependent angular momentum loss law inferred from open cluster studies produces a nearly flat plateau, unlike previous models which exhibited a downwards curvature for hotter temperatures in the Li Teff plane. The overall depletion factor for the plateau stars is sensitive primarily to the solar initial angular momentum used in the calibration for the mixing diffusion coefficients. The Li/Li depletion ratio is also examined. We find that the dispersion in the plateau and the Li/Li depletion ratio scale with the absolute Li depletion in the plateau and we use observational data to set bounds on the Li depletion in main sequence halo stars. A maximum of 0.4 dex depletion is set by the observed dispersion and Li/Li depletion ratio and a minimum of 0.2 dex depletion is required by both the presence of highly overdepleted halo stars and consistency with the solar and open cluster Li data. The cosmological implications of these bounds on the primordial abundance of Li are discussed.