The C-12/C-13 isotope ratio of the interstellar medium in the neighborhood of the sun

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.