Lithium and Lithium Depletion in Halo Stars on Extreme Orbits

We have determined Li abundances in 55 dwarfs and subgiants that are metal-poor (-3.6 < [Fe/H] < -0.7) and have extreme orbital kinematics. Our purpose is to examine the Li abundance in the Li plateau stars and its decrease in low-temperature, low-mass stars. For the stars in our sample we have determined chemical profiles given in 2002 by Stephens & Boesgaard. The Li observations are primarily from the echelle spectrograph on the 10 m Keck I telescope, with HIRES covering 4700-6800 Å with a spectral resolution of ~48,000. The spectra have high signal-to-noise ratios, from 70 to 700 pixel-1, with a median of 140. The Li I resonance doublet was detected in 42 of the 55 stars. Temperatures were found spectroscopically by Stephens & Boesgaard. Abundances or upper limits were determined for all stars, with typical errors of 0.06 dex. Corrections for the deviations from nonlocal thermodynamical equilibrium for Li in the stellar atmospheres have been made, which range from -0.04 to +0.11 dex. Our 14 dwarf and turnoff stars on the Li plateau with temperatures greater than 5700 K and [Fe/H] < -1.5 give A(Li) = log N(Li)/N(H) + 12.00 of 2.215 ± 0.110, consistent with earlier results. We find a dependence of the Li abundance on metallicity as measured by [Fe/H] and the Fe-peak elements Cr and Ni, with a slope of ~0.18. We have examined the possible trends of A(Li) with the chemical abundances of other elements and find similar dependences of A(Li) with the α-elements Mg, Ca, and Ti. These slopes are slightly steeper at ~0.20, resulting from an excess in [α/Fe] with decreasing [Fe/H]. For the n-capture, rare-earth element Ba, we find a relation between A(Li) and [Ba/H] that has a shallower slope of ~0.13; over a range of 2.6 dex in [Ba/H], the Li abundance spans only a factor of 2. We have also examined the possible trends of A(Li) with the characteristics of the orbits of our halo stars. We find no trends in A(Li) with kinematic or dynamic properties. For the stars with temperatures below the Li plateau, there are several interesting results. The group of metal-poor stars possess, on average, more Li at a given temperature than metal-rich stars. When we divide the cool stars into smaller subsets with similar metallicities, we find trends of A(Li) with temperature for the different metallicity groups. The decrease in A(Li) sets in at hotter temperatures for the higher metallicity stars than for the lower metallicity stars. The increased Li depletion in cooler stars could be a result of the increased action of convection, since cooler stars have deeper convection zones. This would also make it easier for additional mixing mechanisms, such as those induced by rotation, to have a greater effect in cooler stars. Since the model depth of the convection zone is almost independent of metallicity at a given effective temperature, the apparent metallicity dependence of the Li depletion in our data may be pointing to subtle but poorly understood mixing effects in low-mass halo dwarfs. Predictions for Li depletion from standard and nonstandard models seem to underestimate the degree of depletion inferred from the observations of the cool stars.