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