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

Abstract

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 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 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 ζOph 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. © ESO 2005.