Interstellar 12 C / 13 C ratios through CH + λλ 3957 , 4232 absorption in local clouds : incomplete mixing in the ISM ⋆

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 + λλ3957.7,4232.3 and 13 CH + λλ3958.2,4232.0 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 ζOph, 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 ± 1.83, while the weighted dispersion of our measurements is 12.7, giving a 6.9 σ scatter. Thus we report on a 6.9 σ detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ∼100 pc: The observed variations in 13 C/ 12 C are the first significant detection of chemical heterogeneity in the local ISM.