Interstellar 12 C / 13 C from CH + absorption lines : Results from an extended survey ⋆

Context. The 12C/13C isotope ratio in the interstellar medium (ISM), and its evolution with time, is an important tracer of stellar yields. Spatial variations of this ratio can be used to study mixing in the ISM. Aims. We want to determine this ratio and its spatial variations in the local ISM from CH absorption lines in the optical towards early-type stars. The aim is to determine the average value for the local ISM and study possible spatial variations. Methods. We observed a large number of early-type stars with Feros to extend the sample of suitable target stars for CH isotope studies. The best suited targets were observed with Uves with higher signal-to-noise ratio and spectral resolution to determine the isotope ratio from the interstellar CH lines. This study significantly expands the number of 13CH+ detections. Results. We find an average ratio of 〈R〉 = 76.27± 1.94 or, for f = 1/R, 〈 f 〉 = (120.46± 3.02)× 10−4 . The scatter in f is 6.3×σ(〈 f 〉). This findings strengthens the case for chemical inhomogeneity in the local ISM, with important implications for the mixing in the ISM. Given the large scatter, the present-day value in the ISM is not significantly larger than the solar value, which corresponds to the local value 4.5 Gyr ago.