Lithium isotope ratios in halo stars. III.

New, high-quality echelle spectra of four halo stars with metallicities in the range 2.0 less than or equal to [Fe/H] less than or equal to -0.9 have been obtained in the region of the Li I lambda 6707 lines. Upper limits on the Li isotopic ratio ranging from Li-6/Li-7 less than or equal to 0.02 to Li-6/Li-7 less than or equal to 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 Li-6/Be ratios seen in HD 84937 and BD +26 degrees 3578 indicate that Li-6 production occurred primarily via the He-4(alpha, H-2)Li-6 reaction between cosmic rays and the interstellar gas, at [Fe/H] less than or similar to -2.3. The much lower Li-6/Be upper limits measured in the much more metal-rich stars HD 132475 and HD 134169 appear to require either (1) large upward Li depletion corrections for these two stars or (2) a transition at [Fe/H] greater than or similar to -2.3 to a much lower Li-6/Be Galactic production ratio, or some combination of both effects.