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 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 13CO angular area
(A13COβ) generally increases with that of 12CO (A12COβ), and the ratio of A13COβ to A12COβ is 0.38 by
linear fitting. We find that the 12CO and 13CO flux are tightly
correlated as F13COβΒ =Β 0.17Β F12COβ with both fluxes
calculated within the 13CO-bright region. This indicates that the
abundance X13COβ is a constant to be 6.5β0.5+0.1βΓ10β7 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 FC18OβΒ =Β 0.11Β F13COβ with both fluxes calculated within
C18O-bright region, which indicates that the abundance ratios X13COβ/XC18Oβ stays the same value 9.7β0.8+0.6β across the
molecular clouds under LTE assumption. The linear relationships of F12COβ vs. F13COβ and F13COβ vs. FC18Oβ
hold not only for the 13CO-bright region or C18O-bright region, but
also for the entire molecular cloud scale with lower flux ratio. The abundance
ratio X13COβ/XC18Oβ inside clouds shows a strong
correlation with column density and temperature. This indicates that the
X13COβ/XC18Oβ is dominated by a combination of chemical
fractionation, selectively dissociation, and self-shielding effect inside
clouds.Comment: 11 pages, 16 figures, 1 table, accepted by A