The molecular clouds in a section of the third Galactic quadrant: observational properties and chemical abundance ratio between CO and its isotopologues

Abstract

We compare the observational properties between ${}^{12}$CO, ${}^{13}$CO, and C${}^{18}$O and summarize the observational parameters based on 7069 clouds sample from the Milky Way Imaging Scroll Painting (MWISP) CO survey in a section of the third Galactic quadrant. We find that the ${}^{13}$CO angular area ($A_{{}^{13}\mathrm{CO}}$) generally increases with that of ${}^{12}$CO ($A_{{}^{12}\mathrm{CO}}$), and the ratio of $A_{{}^{13}\mathrm{CO}}$ to $A_{{}^{12}\mathrm{CO}}$ is 0.38 by linear fitting. We find that the ${}^{12}$CO and ${}^{13}$CO flux are tightly correlated as $F_{{}^{13}\mathrm{CO}}=0.17F_{{}^{12}\mathrm{CO}}$ with both fluxes calculated within the ${}^{13}$CO-bright region. This indicates that the abundance $X_{{}^{13}\mathrm{CO}}$ is a constant to be 6.5${}_{-0.5}^{+0.1}$ $\times 10^{-7}$ for all samples under assumption of local thermodynamic equilibrium (LTE). Additionally, we observed that the X-factor is approximately constant in large sample molecular clouds. Similarly, we find $F_{{\mathrm{C}}^{18}\mathrm{O}}=0.11F_{{}^{13}\mathrm{CO}}$ with both fluxes calculated within C${}^{18}$O-bright region, which indicates that the abundance ratios $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$ stays the same value 9.7${}_{-0.8}^{+0.6}$ across the molecular clouds under LTE assumption. The linear relationships of $F_{{}^{12}\mathrm{CO}}$ vs. $F_{{}^{13}\mathrm{CO}}$ and $F_{{}^{13}\mathrm{CO}}$ vs. $F_{{\mathrm{C}}^{18}\mathrm{O}}$ hold not only for the ${}^{13}$CO-bright region or C${}^{18}$O-bright region, but also for the entire molecular cloud scale with lower flux ratio. The abundance ratio $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$ inside clouds shows a strong correlation with column density and temperature. This indicates that the $X_{{}^{13}\mathrm{CO}}/X_{{\mathrm{C}}^{18}\mathrm{O}}$ is dominated by a combination of chemical fractionation, selectively dissociation, and self-shielding effect inside clouds.

11 pages, 16 figures, 1 table, accepted by AJ