THE ~(12)C/~(13)C ISOTOPIC RATIO IN PHOTODISSOCIATED GAS IN M42

摘要

We have observed the 158 μm ~2P_(3/2) ← ~2P_(1/2) fine-structure line of ~(12)C II simultaneously with the F = 2 ← 1 and F = 1 ← 0 hyperfine components of this transition in ~(13)C II in the Orion photodissociation region near θ~1C. The line profiles were fully resolved using a heterodyne spectrometer with 0.5 km s~(-1) resolution. The relative intensities of these lines give a ~(12)C/~(13)C isotopic ratio of R = 58(_(-5)~(+6)) for the most probable ~(12)C II peak optical depth τ = 1.3. The constrained range of τ(~(12)C II) between 1.0 and 1.4 corresponds to a range of ~(12)C/~(13)C between 52 and 61. The most probable value of 58 agrees very well with the value obtained from a relationship between the isotopic ratio and Galactocentric distance derived from CO measurements, but it is lower than the specific value, 67 ± 3, obtained for Orion from CO data. An isotopic ratio as low as 43, as previously suggested based on optical absorption measurements of the local interstellar medium, is excluded by the C II data at about the 2 σ level.