7Li/6Li Ratio in Distant Interstellar Media and Primordial 7Li—Implementation in Big‐Bang Cosmology and Galactic Chemical Evolution—

We have detected the isotopic abundance ratio of 7Li/6Li in the interstellar media (ISMs) along the line of sight to HD169454 and HD250290 using High Dispersion Spectrograph equipped with the Subaru Telescope. We also observed ζ Oph for the comparison purpose with the previous data. The first background star HD169454 is at the distance 1.7 kpc inward from the solar sysytem in the Galactic plane, and the absorbing clouds are estimated to locate at 0.125 kpc⩽d⩽0.7 kpc and/or 1 kpc⩽d⩽1.7 kpc. If the observed 7Li/6Li ratio is for the gas cloud at the larger distance 1 kpc⩽d⩽1.7 kpc, this could be the first observation of the ISM 7Li/6Li ratio beyond the solar neighborhood. As for the second background star HD250290, the distance from the solar system is uncertain, 2.2±0.2 kpc or 0.439−0.147+0.446 kpc. The observed abundance ratios turn out to be 7Li/6Li = 8.1−1.8+3.6, and 6.3−1.7+3.0 for HD169454 and HD250290, respectively. These values are in reasonable agreement with those observed previously in the ISMs of...