2014A&A...564A..68S

We derive physical properties such as the optical depths and the column densities of ¹³CO and C¹⁸O to investigate the relationship between the far ultraviolet (FUV) radiation and the abundance ratios between ¹³CO and C¹⁸O. We have carried out wide-field (0.4deg²) observations with an angular resolution of 25.8'' (∼0.05pc) in ¹³CO (J=1-0) and C¹⁸O (J=1-0) toward the Orion-A giant molecular cloud using the Nobeyama 45m telescope in the on-the-fly mode. Overall distributions and velocity structures of the ¹³CO and C¹⁸O emissions are similar to those of the ¹²CO (J=1-0) emission. The optical depths of the ¹³CO and C¹⁸O emission lines are estimated to be 0.05<τ(¹³CO)<1.54 and 0.01<τ(C¹⁸O)<0.18, respectively. The column densities of the ¹³CO and C¹⁸O emission lines are estimated to be 0.2x10¹⁶<N(¹³CO)<3.7x10¹⁷cm^–2 and 0.4x10¹⁵<N(C¹⁸O)<3.5x10¹⁶cm^–2, respectively. The abundance ratios between ¹³CO and C¹⁸O, X13^CO/X_C18^O_, are found to be 5.7-33.0. The mean value of X(¹³CO)/X(C¹⁸O) in the nearly edge-on photon-dominated regions is found to be 16.47±0.10, which is a third larger than that the solar system value of 5.5. The mean value of X(¹³CO)/X(C¹⁸O) in the other regions is found to be 12.29±0.02. The difference of the abundance ratio is most likely due to the selective FUV photodissociation of C¹⁸O.