2005A&A...441..181C

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 ¹²CH⁺λλ3957.7,4232.3 and ¹³CH⁺λλ3958.2,4232.0 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 ζ Oph, 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 78.27±1.83, 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: R=74.7±2.3 in the ζ 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 ¹³C/¹²C are the first significant detection of chemical heterogeneity in the local ISM.ME ZET OPH CLOUD