Detection of the [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]2[/TINF] → [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF] Submillimeter Transition of [TSUP]13[/TSUP]C [CSC]i[/CSC] in the Interstellar Medium: Implication for Chemical Fractionation

We report the first detection of the submillimeter emission from the 13 C isotope of atomic carbon in the interstellar medium. The F =5/2-3/2 component of the 3 P 2 - 3 P 1 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 12 C-to- 13 C isotopic abundance ratio is 58±12 corrected for opacity of the 12 C I line and the fractional intensity of the 13 C I hyperfine component (60%). This is in agreement with the value for the equivalent ratio in C + . In comparison, our measurement of the C 18 O-to- 13 C 18 O 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 12 C-to- 13 C abundance ratio is particularly sensitive to chemical fractionation effects. If 13 C + is preferentially incorporated into 13 CO at cloud edges, there will be a dramatic reduction in the abundance of 13 C. This is contrary to our observations, implying that the importance of chemical fractionation is small or is compensated for by isotopic-selective photodissociation of 13 CO in this region with a large UV illumination.