Detection of the 3P2→3P1 submillimeter transition of 13C I in the interstellar medium:: Implication for chemical fractionation

We report the first detection of the submillimeter emission from the C-13 isotope of atomic carbon in the interstellar medium. The F = 5/2-3/2 component of the P-3(2)-P-3(1) transition was observed with the Caltech Submillimeter Observatory in a region similar to 4' S of Orion IRc2, near the western end of the Orion Bar. The C-12-to-C-13 isotopic abundance ratio is 58 +/- 12 corrected for opacity of the C-12 I line and the fractional intensity of the C-13 I hyperfine component (60%). This is in agreement with the value for the equivalent ratio in C+. In comparison, our measurement of the (CO)-O-18-to-(CO)-C-13-O-18 ratio from observations of 2-1 and 3-2 lines toward the same position gives a value of 75 +/- 9. Photodissociation region models predict that the C-12-to-C-13 abundance ratio is particularly sensitive to chemical fractionation effects. If C-13(+) is preferentially incorporated into (CO)-C-13 at cloud edges, there will be a dramatic reduction in the abundance of C-13. This is contrary to our observations, implying that the importance of chemical fractionation is small or is compensated for by isotopic-selective photodissociation of (CO)-C-13 in this region with a large UV illumination.