C-12 C-13 ISOTOPE RATIO IN THE LOCAL INTERSTELLAR-MEDIUM FROM OBSERVATIONS OF (CO)-C-13-O-18 IN MOLECULAR CLOUDS/

The C-12/C-13 isotope ratio is studied in the solar neighborhood from observations of the (CO)-C-12-O-18 and (CO)-C-13-O-18 J = 1-0 transiti ons in four interstellar clouds located within 500 pc. Three of these, B5, B335, and L183 (L134N), are dark clouds not associated with stron g H II regions, and the fourth is at a position in Orion A relatively distant from the H II region associated with the KL source. The C-12/C -13 ratio in these sources (after corrections for a small amount of ra diative trapping) ranges from 57 to 74, and its weighted average is 62 +/- 4, with thermal noise and line formation uncertainties contributi ng about equally to the probable error. This ratio is consistent with the value we previously derived from the (CO)-C-12-O-18/(CO)-C-13-O-18 Galactic gradient, as well as with the most likely range, 60-70, dete rmined from CH+ absorption measurements in the diffuse cloud zeta Oph. Thus the most reliable CO and CH+ results are consistent within their errors. The resulting value indicates moderate chemical evolution in the local solar neighborhood since the formation of the solar system, in good agreement with model prediction of the current carbon ratio in the local interstellar medium.