High abundance ratio of ¹³CO to C¹⁸O toward photon-dominated regions in the Orion-A giant molecular cloud

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Aims. 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. Method. We have carried out wide-field (0.4 deg²) observations with an angular resolution of 25.8 arcsec (0.05 pc) in ¹³CO (J=1--0) and C¹⁸O (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 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 0.2 × 10¹⁶ < N ¹³CO < 3.7 × 10¹⁷ cm⁻² and 0.4 × 10¹⁵ < N C¹⁸O < 3.5 × 10¹⁶ cm⁻², respectively. The abundance ratios between ¹³CO and C¹⁸O, 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 C¹⁸O.