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
Abstract
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, X13CO/XC18O, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of X13CO/XC18O 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 X13CO/XC18O decrease from 17 to 10 with the median of the column densities of the clouds at the column density of NC18O 1 × 1015 cm-2 or visual extinction of AV 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.