Lithium and Lithium Depletion in Halo Stars on Extreme Orbits

Abstract

We have determined Li abundances in 55 dwarfs and subgiants that aremetal-poor (-3.6{\lt}[Fe/H]{\lt}-0.7) and have extreme orbital kinematics.Our purpose is to examine the Li abundance in the Li plateau stars andits decrease in low-temperature, low-mass stars. For the stars in oursample we have determined chemical profiles given in 2002 by Stephens{\amp} Boesgaard. The Li observations are primarily from the echellespectrograph on the 10 m Keck I telescope, with HIRES covering 4700-6800{Å} with a spectral resolution of ~48,000. The spectra have highsignal-to-noise ratios, from 70 to 700 pixel^{-1}, with a medianof 140. The Li I resonance doublet was detected in 42 of the 55 stars.Temperatures were found spectroscopically by Stephens {\amp} Boesgaard.Abundances or upper limits were determined for all stars, with typicalerrors of 0.06 dex. Corrections for the deviations from nonlocalthermodynamical equilibrium for Li in the stellar atmospheres have beenmade, which range from -0.04 to +0.11 dex. Our 14 dwarf and turnoffstars on the Li plateau with temperatures greater than 5700 K and[Fe/H]{\lt}-1.5 give A(Li)=logN(Li)/N(H)+12.00 of 2.215+/-0.110,consistent with earlier results. We find a dependence of the Liabundance on metallicity as measured by [Fe/H] and the Fe-peak elementsCr and Ni, with a slope of ~0.18. We have examined the possible trendsof A(Li) with the chemical abundances of other elements and find similardependences of A(Li) with the {α}-elements Mg, Ca, and Ti. Theseslopes are slightly steeper at ~0.20, resulting from an excess in[{α}/Fe] with decreasing [Fe/H]. For the n-capture, rare-earthelement Ba, we find a relation between A(Li) and [Ba/H] that has ashallower slope of ~0.13 over a range of 2.6 dex in [Ba/H], the Liabundance spans only a factor of 2. We have also examined the possibletrends of A(Li) with the characteristics of the orbits of our halostars. We find no trends in A(Li) with kinematic or dynamic properties.For the stars with temperatures below the Li plateau, there are severalinteresting results. The group of metal-poor stars possess, on average,more Li at a given temperature than metal-rich stars. When we divide thecool stars into smaller subsets with similar metallicities, we findtrends of A(Li) with temperature for the different metallicity groups.The decrease in A(Li) sets in at hotter temperatures for the highermetallicity stars than for the lower metallicity stars. The increased Lidepletion in cooler stars could be a result of the increased action ofconvection, since cooler stars have deeper convection zones. This wouldalso make it easier for additional mixing mechanisms, such as thoseinduced by rotation, to have a greater effect in cooler stars. Since themodel depth of the convection zone is almost independent of metallicityat a given effective temperature, the apparent metallicity dependence ofthe Li depletion in our data may be pointing to subtle but poorlyunderstood mixing effects in low-mass halo dwarfs. Predictions for Lidepletion from standard and nonstandard models seem to underestimate thedegree of depletion inferred from the observations of the cool stars.