Interstellar 12C/13C ratios through CH+ λλ3957,4232 absorption in local clouds:: incomplete mixing in the ISM

The C-12/C-13 isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM. (CH+)-C-12/(CH+)-C-13 is not affected by interstellar chemistry, and is the most secure way of measuring C-12/C-13 in the diffuse ISM. R = C-12/C-13 is 90 in the solar system. Previous measurements of (CH+)-C-12 lambda lambda 3957.7,4232.3 and (CH+)-C-13 lambda lambda 3958.2,4232.0 absorption toward nearby stars indicate some variations in C-12/C-13, 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 C-12/C-13 measurements. The weighted C-12/C-13 ratio in the local ISM is 78.27 +/- 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 similar to 100 pc: R = 74.7 +/- 2.3 in the zeta Oph cloud, while R = 88.6 +/- 3.0 toward HD 152235 in the Lupus clouds, R = 62.2 +/- 5.3 towards HD 110432 in the Coalsack, and R = 98.9 +/- 10.1 toward HD 170740. The observed variations in C-13/C-12 are the first significant detection of chemical heterogeneity in the local ISM.