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

Abstract

We report the first detection of the submm emission from the 13 C isotope of atomic carbon in the ISM. The F = 5 / 2 − 3 / 2 component of the 3 P 2 − 3 P 1 transition was observed with the CSO 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. PDR 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 photo-dissociation of 13 CO in this region with a large UV illumination.

13C I in high-mass star-forming clouds

“…As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of 13 C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of 13 CO, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al (1998). …”

Photodissociation regions in the interstellar medium of galaxies

“…The recent observation of the 13 CI 3 P 2 → 3 P 1 transition in the Orion Bar indicates a high column of 13 C there, which suggests little chemical fractionation of 13 C into 13 CO, possibly due to isotope-selective photodissociation of 13 CO in the PDR (Keene, Schilke, et al, 1998).…”

“…CO shielding functions have been tabulated by van and by Lee et al (1996). For low ratios of G 0 /n, the effects of CO self-shielding can lead to isotopic fractionation effects at the borders of clouds, where the rarer CO isotopes are preferentially photodissociated (Bally and Langer, 1982;Chu and Watson, 1983;Glassgold et al, 1985;Viala et al, 1988;Gredel et al, 1994;Warin et al, 1996;Keene, Schilke, et al 1998). However, the location of the C ϩ /C/CO transition in bright PDRs is largely governed by dust extinction.…”

Carbon fractionation in photo-dissociation regions

“…Following Wakelam & Herbst (2008) we assume a standard elemental abundance of 13 C/ 12 C of 67 in the solar neighbourhood. Observing 13 [C i] and 13 C 18 O in the Orion Bar Keene et al (1998) found little evidence for chemical fractionation. Their observations showed a slight enhancement of C 18 O/ 13 C 18 O = 75 and no enhancement of [ 13 Ci]/ [Ci] relative to the standard elemental abundance while the chemical models predicted the opposite.…”

“…Carbon fractionation in molecular and diffuse clouds has been discussed systematically by Keene et al (1998) and Liszt (2007). Following Wakelam & Herbst (2008) we assume a standard elemental abundance of 13 C/ 12 C of 67 in the solar neighbourhood.…”