We report measurements of the ^ { 12 } C/ ^ { 13 } C abundance ratio in the three galactic regions G 333.0-0.4 , NGC 6334 A and G 351.6-1.3 from observations of the ^ { 12 } C i ^ { 3 } P _ { 2 } \rightarrow ^ { 3 } P _ { 1 } transition and the hyperfine components of the corresponding ^ { 13 } C i transition near 809 GHz .
These transitions were observed simultaneously with the CO 7–6 line emission at 806 GHz with the AST/RO telescope located at the South Pole .
From a simultaneous fit to the ^ { 12 } C i ^ { 3 } P _ { 2 } \rightarrow ^ { 3 } P _ { 1 } transition and the HF components of the corresponding ^ { 13 } C i transition and an independent estimate of an upper limit to the optical depth of the ^ { 12 } C i emission we determine intrinsic ^ { 12 } C i / ^ { 13 } C i column density ratios of 23 \pm 1 for G 333.0-0.4 , 56 \pm 14 for NGC 6334 A and 69 \pm 12 for G 351.6-1.3 .
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 ) .