The C-12/C-13 isotope ratio of the interstellar medium in the neighborhood of the sun
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Abstract
The authors have obtained the 12C/13C isotope ratio of the interstellar medium (ISM) in the neighborhood of the Sun, from optical observations of CH+ toward the stars ξ Per, P Cyg, 20 Tau, and 23 Tau. As a result of this work, and earlier data toward ζ Oph, it is found that the local interstellar medium 12CH+/13CH+ abundance ratios toward five stars and in four different lines of sight are uniform within 12%. The similarity among the 12CH+/13CH+ ratio determined in four interstellar clouds possessing different physical conditions precludes the possibility that fractionation or selective photodissociation of CH+ is taking place in these regions. This work thus provides reliable evidence in favor of the existence of a locally homogeneous ISM, and a weighted mean 12C/13C ratio of 43±4 is calculated from the measured values.