SIMBAD references

We report measurements of the ¹²C/¹³C 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 ¹²CI ³P₂->³P₁ transition and the hyperfine components of the corresponding ¹³CI 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 ¹²CI ³P₂->³P₁ transition and the HF components of the corresponding ¹³CI transition and an independent estimate of an upper limit to the optical depth of the ¹²CI emission we determine intrinsic ¹²CI/¹³CI 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 ¹³C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of ¹³CO, 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. (1998ApJ...494L.107K).