$^{13}$C I in high-mass star-forming clouds

We report measurements of the C/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 Ci P2 →P1 transition and the hyperfine components of the corresponding Ci 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 Ci P2 →P1 transition and the HF components of the corresponding Ci transition and an independent estimate of an upper limit to the optical depth of the Ci emission we determine intrinsic Ci/Ci 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 photon dominated, we argue that the apparent enhancement in the abundance of C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of 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).