(C-12)/(C-13) isotope ratio in the local interstellar medium from observations of (C-13)(O-18) in molecular clouds

The paper examines the (C-12)/(C-13) isotope ratio in the solar neighborhood on the basis of observations of the (C-12)(O-18) and (C-13)(O-18) J = 1-0 transitions in four interstellar clouds located within 500 pc. The (C-12)/(C-13) ratio in these sources ranges from 57 to 74, and its weighted average is 62 +/- 4, with thermal noise and line formation uncertainties contributing about equally to the probable error. These values indicate moderate chemical evolution in the local star neighborhood since the formation of the solar system, in good agreement with model prediction of the current carbon ratio in the local interstellar medium.