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

The 12C/13C isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM. 12CH+/13CH+ is not affected by interstellar chemistry, and is the most secure way of measuring 12C/13C in the diffuse ISM. R=12C/13C is 90 in the solar system. Previous measurements of 12CH3957.7,4232.3 and 13CH3958.2,4232.0 absorption toward nearby stars indicate some variations in 12C/13C, 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 12C/13C measurements. The weighted 12C/13C ratio in the local ISM is , while the weighted dispersion of our measurements is 12.7, giving a 6.9σ scatter. Thus we report on a 6.9σ detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ~100 pc: in the ζOph cloud, while toward HD 152235 in the Lupus clouds, towards HD 110432 in the Coalsack, and toward HD 170740. The observed variations in 13C/12C are the first significant detection of chemical heterogeneity in the local ISM.