The ^ { 12 } C/ ^ { 13 } C isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM . ^ { 12 } CH ^ { + } / ^ { 13 } CH ^ { + } is not affected by interstellar chemistry , and is the most secure way of measuring ^ { 12 } C/ ^ { 13 } C in the diffuse ISM . R = ^ { 12 } C/ ^ { 13 } C is 90 in the solar system .
Previous measurements of ^ { 12 } CH ^ { + } \lambda \lambda 3957.7,4232.3 and ^ { 13 } CH ^ { + } \lambda \lambda 3958.2,4232.0 absorption toward nearby stars indicate some variations in ^ { 12 } C/ ^ { 13 } C , with values ranging from 40 to 90 suggesting inefficient mixing .
Except for the cloud toward \zeta Oph , these R values are strongly affected by noise .
With UVES on the VLT we have improved on the previous interstellar ^ { 12 } C/ ^ { 13 } C measurements .
The weighted ^ { 12 } C/ ^ { 13 } C ratio in the local ISM is 78.27 \pm 1.83 , while the weighted dispersion of our measurements is 12.7 , giving a 6.9 \sigma scatter .
Thus we report on a 6.9 \sigma detection of 16.2 % root-mean-square variations in the carbon isotopic ratio on scales of \sim 100 pc : R = 74.7 \pm 2.3 in the \zeta Oph cloud , while R = 88.6 \pm 3.0 toward HD152235 in the Lupus clouds , R = 62.2 \pm 5.3 towards HD110432 in the Coalsack , and R = 98.9 \pm 10.1 toward HD170740 .
The observed variations in ^ { 13 } C/ ^ { 12 } C are the first significant detection of chemical heterogeneity in the local ISM .