THE INTERSTELLAR LI-7/LI-6 ISOTOPE RATIO TOWARD ZETA-OPHIUCHI AND ZETA-PERSEI

We report high S/N (> 2000), high-resolution (almost-equal-to 1 km s-1) observations of the interstellar Li I absorption lines toward the stars zeta Oph and zeta Per. Our Li I line profiles indicate the presence of both the Li-7 (lambda(rest) = 6707.761, 6707.912 angstrom) and Li-6 (6707.921, 6708.072 angstrom) doublets in these two sightlines. The resultant best-fit values (using a single-component profile) for the interstellar Li-7/Li-6 isotope ratio are 6.8 (+ 1.4, - 1.7) toward zeta Oph and 5.5 (+ 1.3, - 1.1) toward zeta Per. It is clear from our data that the Li-7/Li-6 ratio toward zeta Oph and zeta Per is less than the meteoritic value (12.6) representative of the early solar system. This result does not support the lower limit of 25 set for the interstellar Li-7/Li-6 ratio by Ferlet & Dennefeld from their Li I observations toward zeta Oph. However, our findings agree much better with the recent Li-7/Li-6 measurement of 12.5 (+ 4.3, - 3.4) toward rho Oph by Lemoine et al. Since current models of Galactic chemical evolution predict that the Li-7/Li-6 ratio should increase with time, these new results suggest that the production of Li-6 is being enhanced through cosmic-ray spallation or some other process.