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 (CO)-C-12 (J = 1-0), (CO)-C-13 (J = 1-0), and (CO)-O-18 (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 (CO)-C-13. 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 (CO)-C-13 and (CO)-O-18, X-13CO/X-C18O, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of X-13CO/ X-C18O 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-13CO/X-C18O decrease from 17 to 10 with the median of the column densities of the clouds at the column density of N-C18O greater than or similar to 1 x 10(15) cm(-2) or visual extinction of A(V) greater than or similar to 3 mag under the strong far-ultraviolet (FUV) environment of G(0) > 10(3), whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of (CO)-O-18 is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.