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 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 () generally increases with that of 12CO (), and the ratio of to is 0.38 by linear fitting. We find that the 12CO and 13CO flux are tightly correlated as with both fluxes calculated within the 13CO-bright region. This indicates that the abundance is a constant as 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 with both fluxes calculated within the C18O-bright region, which indicates that the abundance ratio stays at the same value of across the molecular clouds under the LTE assumption. The linear relationships of versus and versus 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 inside the clouds shows a strong correlation with the column density and temperature. This indicates that the ratio is dominated by a combination of chemical fractionation, selective dissociation, and the self-shielding effect inside the clouds.