LITHIUM PROCESSING IN HALO DWARFS, AND T-EFF, [FE H] CORRELATIONS ON THE SPITE PLATEAU/

We present new Li data for seven halo turnoff stars chosen to test for diffusion. These are combined with data from the literature, and new effective temperatures and Li abundances are computed for the entire s et on uniform temperature and abundance scales, We conclude that the e ffects expected of diffusion are not obvious in warm halo dwarfs, and uninhibited diffusion is unlikely to result in an initial Li abundance more than 0.1 dex higher than that inferred from nondiffusive models. (Other mechanisms for depleting Li, possibly by substantially larger amounts, are still possible.) We consider the ongoing debate concernin g the existence of correlations between the Li abundances for metal-po or stars and T-eff and overall metallicity, [Fe/H]. Molaro, Primas, & Bonafacio argue that previously reported slopes in the plateau as a fu nction of these two variables (by Thorburn and Norris, Ryan, & Stringf ellow) disappear when a subset of stars with temperatures based on Bal mer line profiles is adopted. Upon closer examination of the Molaro et al. data and our own newly expanded data, we find that these correlat ions persist, but several points are worth noting: (1) suspected subgi ants should be eliminated from the sample, (2) metallicity trends are evident only when stars of a wide range of metal abundances are includ ed in the samples, especially the most metal-poor stars, (3) the tests must be performed in a multiple-regression environment (i.e., not whe n T-eff or [Fe/H] is considered the only independent variable), and (4 ) the results survive when robust regression methods are applied. Our current best estimate of the mean Li abundance as a function of T-eff and [Fe/H] is +0.0408(+/-0.0052)T-eff/100+0.111(+/-0.018)[Fe/H]. The s lopes of this relationship are consistent, within expected errors, wit h the results of Thorburn and Norris et al. The reported correlations appear to be real, in contradiction to the claim of Molaro et al. We i dentify rare cases of well-observed stars with similar temperatures an d metallicities which cannot have the same Li abundance; the contrast between G64-12, G64-37, and CD-33 degrees 1173 provides the best examp le. However, we defer our main discussion on a possible intrinsic spre ad in the Spite plateau to a separate paper.