LITHIUM ISOTOPE RATIOS IN HALO STARS - II

New, high-precision observations of the Li I lambda 6707 blend in the spectra of five halo stars with metallicities in the range -2.0 less t han or equal to [Fe/H] less than or equal to -1.0 yield no positive de tections of the lighter isotope at upper limits ranging from Li-6/Li-7 less than or equal to 0.02 to Li-6/Li-7 less than or equal to 0.13. I n determining these limits, we removed two constraints that were previ ously imposed during comparison of the observed and synthetic line pro files. A reanalysis of our previous similar observations of six other halo stars with -2.9 less than or equal to [Fe/H] less than or equal t o -1.1 was also carried out with this same procedure. The reanalysis c onfirms our earlier detection of Li-6 in HD 84937 with an isotopic rat io Li-6/Li-7 = 0.08 +/- 0.04 (1 sigma), in agreement with Smith, Lambe rt, & Nissen, but converts our reported detection of Li-6 in HD 201891 to a comparable upper limit only, Li-6/Li-7 less than or equal to 0.0 55. Among the 12 halo stars examined for Li-6 at high sensitivity by o ne or more of three different groups, positive results have been obtai ned only for the hottest dwarf, HD 84937, and possibly for the hottest subgiant, HD 160617, for which Nissen found Li-6/Li-7 = 0.0173 +/- 0. 012 (1 sigma). However, the less sensitive upper limits available for three slightly cooler dwarfs exceed the isotopic ratio at which 6Li is seen in HD 84937. In combination with Be and B abundances measured fo r some of these 12 stars, the very sparse Li-6 data suggest that a pro cess such alpha + alpha reactions at low energies produced much of the Li-6 that was present at [Fe/H] approximate to -2.2, and that at leas t one of the following three assumptions is in error. (1) the Li-6/Be ratio measured in HD 84937 is typical of halo gas at [Fe/H] = -2.2; (2 ) the Galactic Li-6/Be ratio remained approximately invariant at -2.2 less than or similar to [Fe/H] less than or similar to -1.0; and (3) m ost of the original Li-6 in the probable subgiants HD 134169 and HD 20 1891 (with [Fe/H] = -1.0 and -1.1, respectively, and T-e greater than or equal to 5800 K) has survived to the present day, as the observatio ns of Pilachowski, Sneden, & Booth and the calculations of Chaboyer to gether suggest. Alternatively, the low Li-6 fractions required by the upper limits measured in HD 134169 and HD 201891 can be understood if both stars are dwarfs, not subgiants.