(CI)-C-13 in high-mass star-forming clouds

We report measurements of the C-12/C-13 abundance ratio in the three galact ic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the (CI)-C-12 P-3(2) --> P-3(1) transition and the hyperfine components of the corresponding (CI)-C-13 transition near 809 GHz. These transitions were ob served 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 (CI )-C-12 P-3(2) --> P-3(1) transition and the HF components of the correspond ing (CI)-C-13 transition and an independent estimate of an upper limit to t he optical depth of the (CI)-C-12 emission we determine intrinsic (CI)-C-12 /(CI)-C-13 column density ratios of 23 +/- 1 for G 333.0-0.4, 56 +/- 14 for NGC 6334 A and 69 +/- 12 for G 351.6-1.3. As the regions observed are phot on dominated, we argue that the apparent enhancement in the abundance of C- 13 towards G 333.0-0.4 may be due to strong isotope-selective photodissocia tion of (CO)-C-13, outweighing the effects of chemical isotopic fractionati on 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).