Nobeyama 45 m Mapping Observations toward Orion A. II. Classification of cloud structures and variation of the ${}^{13}$CO/C${}^{18}$O abundance ratio due to far-UV Radiation

Abstract

We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field ${}^{12}$CO ($J$ = 1--0), ${}^{13}$CO ($J$ = 1-0), and C${}^{18}$O ($J$ = 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 ${}^{13}$CO. 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 ${}^{13}$CO and C${}^{18}$O, $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$ is also seen within a cloud in both of the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$ decrease from 17 to 10 with the median of the column densities of the clouds at the column density of $N_{{\mathrm{C}}^{18}\mathrm{O}}\gtrsim 1\times 10^{15}$ cm${}^{-2}$ or visual extinction of $A_V\gtrsim 3$ 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 C${}^{18}$O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.

20 pages, 11 figures, accepted in PASJ

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