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 (CO)-C-12, (CO)-C-13, and (CO)-O-18 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 (CO)-C-13 angular area (A(13)CO) generally increases with that of (CO)-C-12 ( A(12)CO), and the ratio of A(13)CO to A(12)CO is 0.38 by linear fitting. We find that the (CO)-C-12 and (CO)-C-13 flux are tightly correlated as F-CO(13) = 0.17 F-CO(12) with both fluxes calculated within the 13CO-bright region. This indicates that the abundance X-CO(13) is a constant as - ' 6.5(+0.1) (-0.5) 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-C(O)18 = 0.11 (13)(F)(CO) with both fluxes calculated within the C18O-bright region, which indicates that the abundance ratio X-CO(13) /(XCO)-O-18 stays at the same value of 9.7+ (+0.6)(-0.8) across the molecular clouds under the LTE assumption. The linear relationships of F-CO(12) versus F-CO(13) and F-CO(13) versus F-C(O)18 hold not only for the (CO)-C-13-bright region and (CO)-O-18-bright region, but also for the entire molecular cloud scale with a lower flux ratio. The abundance ratio X-CO(13) /X-C(O)18 inside the clouds shows a strong correlation with the column density and temperature. This indicates that the ratioX(CO)(13) /X-C(O)18 is dominated by a combination of chemical fractionation, selective dissociation, and the self-shielding effect inside the clouds.

Automated summary

We compared the observational properties of (CO)-C-12, (CO)-C-13, and (CO)-O-18 and summarized the observational parameters based on a sample of 7069 clouds in the Milky Way. We found that the (CO)-C-13 angular area generally increases with (CO)-C-12 and the ratio of A(13)CO to A(12)CO is 0.38. The flux of (CO)-C-12 and (CO)-C-13 are tightly correlated within the 13CO-bright region, indicating a constant abundance of X-CO(13) under the LTE assumption. Additionally, we observed that the abundance ratio X-CO(13) /(XCO)-O-18 stays constant across the molecular clouds under the LTE assumption.