The interstellar ⁷Li/⁶Li isotope ratio toward ζ ophiuchi and ζ persei

We report high S/N (>2000), high-resolution (≈ 1 km s^-1) observations of the interstellar Li I absorption lines toward the stars ζ Oph and ζ Per. Our Li I line profiles indicate the presence of both the ⁷Li (λ_rest = 6707.761, 6707.912 Å) and ⁶Li (6707.921, 6708.072 Å) doublets in these two sightlines. The resultant best-fit values (using a single-component profile) for the interstellar ⁷Li/⁶Li isotope ratio are 6.8 (+1.4, -1.7) toward ζ Oph and 5.5 (+1.3, -1.1) toward ζ Per. It is clear from our data that the ⁷Li/⁶Li ratio toward ζ Oph and ζ 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 ⁷Li/⁶ Li ratio by Ferlet & Dennefeld from their Li I observations toward ζ Oph. However, our findings agree much better with the recent ⁷Li/⁶Li measurement of 12.5 (+4.3, -3.4) toward ρ Oph by Lemoine et al. Since current models of Galactic chemical evolution predict that the ⁷Li/⁶Li ratio should increase with time, these new results suggest that the production of ⁶Li is being enhanced through cosmic-ray spallation or some other process.