Lithium Isotope Ratios in Halo Stars. III.

New, high-quality echelle spectra of four halo stars with metallicities in the range -2.0<=[Fe/H]<=-0.9 have been obtained in the region of the Li I λ6707 lines. Upper limits on the Li isotopic ratio ranging from ⁶Li/⁷Li<=0.02 to ⁶Li/⁷Li<=0.08 are found for HD 76932, HD 218502, and HD 284248. The apparently composite spectrum of the close visual binary HD 219617 proved to be unsuitable for similar analysis. Hipparcos parallaxes are used to evaluate the evolutionary states of the three newly observed stars, along with 15 other stars for which Li isotopic ratios (or upper limits, in all but two cases) were previously observed by three groups. The high ⁶Li/Be ratios seen in HD 84937 and BD +26°3578 indicate that ⁶Li production occurred primarily via the ⁴He(α, ²H)⁶Li reaction between cosmic rays and the interstellar gas, at [Fe/H]<~-2.3. The much lower ⁶Li/Be upper limits measured in the much more metal-rich stars HD 132475 and HD 134169 appear to require either (1) large upward ⁶Li depletion corrections for these two stars or (2) a transition at [Fe/H]>~-2.3 to a much lower ⁶Li/Be Galactic production ratio, or some combination of both effects.