Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field ¹²CO (J = 1-0), ¹³CO (J = 1-0), and C¹⁸O (J = 1-0) observations using the Nobeyama 45 m radio telescope. We identified 78 clouds toward Orion A by applying Spectral Clustering for Interstellar Molecular Emission Segmentation (SCIMES) to the data cube of the column density of ¹³CO. Well-known subregions such as OMC-1, OMC-2/3, OMC-4, OMC-5, NGC 1977, L1641-N, and the dark lane south filament (DLSF) are naturally identified as distinct structures in Orion A. These clouds can also be classified into three groups: the integral-shaped filament, the southern regions of Orion A, and the other filamentary structures in the outer parts of Orion A and the DLSF. These groups show differences in scaling relations between the physical properties of the clouds. We derived the abundance ratio between ¹³CO and C¹⁸O, X_{^{13}CO}/X_{C^{18}O}, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of X_{^{13}CO}/X_{C^{18}O} is also seen within a cloud in both the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of X_{^{13}CO}/X_{C^{18}O} decrease from 17 to 10 with the median of the column densities of the clouds at the column density of N_{C^{18O}} ≳ 1 × 10^{15}cm^-2 or visual extinction of A_V ≳ 3 mag under the strong far-ultraviolet (FUV) environment of G₀ > 10³, whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of C¹⁸O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.