Detection of the ^3P_2 - ^3P_1 Submillimeter Transition of 13CI in the Interstellar Medium: Implication for Chemical Fractionation

Details Detection of the ^3P_2 - ^3P_1 Submillimeter Transition of 13CI in the Interstellar Medium: Implication for Chemical Fractionation Detection of the ^3P_2 - ^3P_1 Submillimeter Transition of 13CI in the Interstellar Medium: Implication for Chemical Fractionation

1997-12-04

arxiv.org/abs/astro-ph/9712071v1

Astronomy and Astrophysics

Astrophysics

Astrophysical Journal Letters in press, 6 pages, 2 figures, AAS emulateapj LaTex style

Scientific paper

10.1086/311164

We report the first detection of the submm emission from the 13C isotope of atomic carbon in the ISM. The F = 5/2 - 3/2 component of the ^3P_2 - ^3P_1 transition was observed with the CSO 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 12CI line and the fractional intensity of the 13CI 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. PDR models predict that the 12C to 13C abundance ratio is particularly sensitive to chemical fractionation effects. If 13C+ 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 photo-dissociation of 13CO in this region with a large UV illumination.

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