High abundance ratio of ${}^{13}$CO to C${}^{18}$O toward photon-dominated regions in the Orion-A giant molecular cloud

Abstract

Aims. We derive physical properties such as the optical depths and the column densities of ${}^{13}$CO and C${}^{18}$O to investigate the relationship between the far ultraviolet (FUV) radiation and the abundance ratios between ${}^{13}$CO and C${}^{18}$O. Method. We have carried out wide-field (0.4 deg${}^2$) observations with an angular resolution of 25.8 arcsec ($\sim$ 0.05 pc) in ${}^{13}$CO ($J$=1--0) and C${}^{18}$O ($J$=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 ${}^{13}$CO and C${}^{18}$O emissions are similar to those of the ${}^{12}$CO ($J$=1--0) emission. The optical depths of the ${}^{13}$CO and C18O emission lines are estimated to be 0.05 $<$ ${\tau }_{{}^{13}\mathrm{CO}}$ $<$ 1.54 and 0.01 $<$ ${\tau }_{{\mathrm{C}}^{18}\mathrm{O}}$ $<$ 0.18, respectively. The column densities of the ${}^{13}$CO and C${}^{18}$O emission lines are estimated to be 0.2 $\times$ 10${}^{16}$ $<$ $N_{{}^{13}\mathrm{CO}}$ $<$ 3.7 $\times$ 10${}^{17}$ cm${}^{-2}$ and 0.4 $\times$ 10${}^{15}$ $<$ $N_{{\mathrm{C}}^{18}\mathrm{O}}$ $<$ 3.5 $\times$ 10${}^{16}$ cm${}^{-2}$, respectively. The abundance ratios between ${}^{13}$CO and C${}^{18}$O, $X_{{}^{13}\mathrm{CO}}$/$X_{{\mathrm{C}}^{18}\mathrm{O}}$, are found to be 5.7 - 33.0. The mean value of $X_{{}^{13}\mathrm{CO}}$/$X_{{\mathrm{C}}^{18}\mathrm{O}}$ in the nearly edge-on photon-dominated regions is found to be 16.47 $\pm$ 0.10, which is a third larger than that the solar system value of 5.5. The mean value of $X_{{}^{13}\mathrm{CO}}$/$X_{{\mathrm{C}}^{18}\mathrm{O}}$ in the other regions is found to be 12.29 $\pm$ 0.02. The difference of the abundance ratio is most likely due to the selective FUV photodissociation of C${}^{18}$O.

11 pages, 9 figures, Accepted to A&A

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