Aims. We derive physical properties such as the optical depths and the column densities of (CO)-C-13 and (CO)-O-18 to investigate the relationship between the far ultraviolet (FUV) radiation and the abundance ratios between (CO)-C-13 and (CO)-O-18.
Methods. We have carried out wide-field (0.4 deg(2)) observations with an angular resolution of 25.8 '' (similar to 0.05 pc) in (CO)-C-13 (J = 1-0) and (CO)-O-18 (J = 1-0) toward the Orion-A giant molecular cloud using the Nobeyama 45 m telescope in the on-the-fly mode.
Results. Overall distributions and velocity structures of the (CO)-C-13 and (CO)-O-18 emissions are similar to those of the (CO)-C-12 (J = 1-0) emission. The optical depths of the (CO)-C-13 and (CO)-O-18 emission lines are estimated to be 0.05 < tau(13CO) < 1.54 and 0.01 < tau(C18O) < 0.18, respectively. The column densities of the (CO)-C-13 and (CO)-O-18 emission lines are estimated to be 0.2 x 10(16) < N-13CO < 3.7 x 10(17) cm(-2) and 0.4 x 10(15) < N-C18O < 3.5 x 10(16) cm(-2), respectively. The abundance ratios between (CO)-C-13 and (CO)-O-18, X-13CO/X-C18O, 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 (CO)-O-18.