The Molecular Clouds in a Section of the Third Galactic Quadrant: Observational Properties and Chemical Abundance Ratio between CO and its Isotopologues

We compare the observational properties between 12CO, 13CO, and C18O and summarize the observational parameters based on a 7069-cloud sample from the Milky Way Imaging Scroll Painting CO survey in a section of the third Galactic quadrant. We find that the 13CO angular area (${A}_{{}^{13}\mathrm{CO}}$) generally increases with that of 12CO (${A}_{{}^{12}\mathrm{CO}}$), and the ratio of ${A}_{{}^{13}\mathrm{CO}}$ to ${A}_{{}^{12}\mathrm{CO}}$ is 0.38 by linear fitting. We find that the 12CO and 13CO flux are tightly correlated as ${F}_{{}^{13}\mathrm{CO}}=0.17\,{F}_{{}^{12}\mathrm{CO}}$ with both fluxes calculated within the 13CO-bright region. This indicates that the abundance ${X}_{{}^{13}\mathrm{CO}}$ is a constant as ${6.5}_{-0.5}^{+0.1}\times {10}^{-7}$ for all samples under the assumption of local thermodynamic equilibrium (LTE). Additionally, we observed that the X-factor is approximately constant in a large sample of molecular clouds. Similarly, we find ${F}_{{{\rm{C}}}^{18}{\rm{O}}}=0.11\,{F}_{{}^{13}\mathrm{CO}}$ with both fluxes calculated within the C18O-bright region, which indicates that the abundance ratio ${X}_{{}^{13}\mathrm{CO}}/{X}_{{{\rm{C}}}^{18}{\rm{O}}}$ stays at the same value of ${9.7}_{-0.8}^{+0.6}$ across the molecular clouds under the LTE assumption. The linear relationships of ${F}_{{}^{12}\mathrm{CO}}$ versus ${F}_{{}^{13}\mathrm{CO}}$ and ${F}_{{}^{13}\mathrm{CO}}$ versus ${F}_{{{\rm{C}}}^{18}{\rm{O}}}$ hold not only for the 13CO-bright region and C18O-bright region, but also for the entire molecular cloud scale with a lower flux ratio. The abundance ratio ${X}_{{}^{13}\mathrm{CO}}/{X}_{{{\rm{C}}}^{18}{\rm{O}}}$ inside the clouds shows a strong correlation with the column density and temperature. This indicates that the ratio ${X}_{{}^{13}\mathrm{CO}}/{X}_{{{\rm{C}}}^{18}{\rm{O}}}$ is dominated by a combination of chemical fractionation, selective dissociation, and the self-shielding effect inside the clouds.