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

Abstract

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 +/- 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 (13)C/(12)C are the first significant detection of chemical heterogeneity in the local ISM.