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 ¹³C isotope of atomic carbon in the interstellar medium. The F=5/2-3/2 component of the ³P₂-³P₁ transition was observed with the Caltech Submillimeter Observatory in a region ~4' S of Orion IRc2, near the western end of the Orion Bar. The ¹²C-to-¹³C isotopic abundance ratio is 58±12 corrected for opacity of the ¹²C I line and the fractional intensity of the ¹³C I hyperfine component (60%). This is in agreement with the value for the equivalent ratio in C⁺. In comparison, our measurement of the C¹⁸O-to-¹³C¹⁸O 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 ¹²C-to-¹³C abundance ratio is particularly sensitive to chemical fractionation effects. If¹³C⁺ is preferentially incorporated into ¹³CO at cloud edges, there will be a dramatic reduction in the abundance of ¹³C. This is contrary to our observations, implying that the importance of chemical fractionation is small or is compensated for by isotopic-selective photodissociation of ¹³CO in this region with a large UV illumination.