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 set on uniform temperature and abundance scales, We conclude that the effects 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 concerning the existence of correlations between the Li abundances for metal-poor stars and T-eff and overall metallicity, [Fe/H]. Molaro, Primas, & Bonafacio argue that previously reported slopes in the plateau as a function of these two variables (by Thorburn and Norris, Ryan, & Stringfellow) disappear when a subset of stars with temperatures based on Balmer line profiles is adopted. Upon closer examination of the Molaro et al. data and our own newly expanded data, we find that these correlations persist, but several points are worth noting: (1) suspected subgiants should be eliminated from the sample, (2) metallicity trends are evident only when stars of a wide range of metal abundances are included in the samples, especially the most metal-poor stars, (3) the tests must be performed in a multiple-regression environment (i.e., not when 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 A(Li)=-0.09(+/-0.30)+0.0408(+/-0.0052)T-eff/100+0.111(+/-0.018)[Fe/H]. The slopes of this relationship are consistent, within expected errors, with 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 identify rare cases of well-observed stars with similar temperatures and metallicities which cannot have the same Li abundance; the contrast between G64-12, G64-37, and CD-33 degrees 1173 provides the best example. However, we defer our main discussion on a possible intrinsic spread in the Spite plateau to a separate paper.