2019ApJS..243...25W

We present analyses of the ¹³CO abundance X_^13CO and the abundance ratio X13CO^/XC^18O of molecular clouds by collecting ¹²CO, ¹³CO, and C¹⁸O (1-0) transitions toward the Galactic anti-center in the ranges 186○ / . 25< l< 195○ / . 25 and -3○ / . 75< b< 2○ / . 75. The 9^○×^6○ / . 5 area contains eight clouds, which can be divided into two types according to X13CO^/XC^18O. Type I clouds, including the chain of Lynds dark clouds, West Front, Gemini OB1 giant molecular cloud (GGMC) 1, and Horn, have lower X13CO^/XC^18O (mean value <10) and lower column density with relatively lower temperature. Type II clouds, including GGMC 2, GGMC 3, GGMC 4, and Swallow, have a higher abundance ratio X13CO^/XC^18O (mean value >10) and higher column density with relatively higher temperature. For all clouds, X_^13CO increases from ∼2.8×10^–7 at the edge to > 1.4×10^–6 in the interior, which is probably due to an opacity effect of ¹²CO. In GGMC 2-4, the mean X13CO^/XC^18O of the molecular gas associated with H II regions and IRAS sources is ∼14, slightly larger than that (∼9) of other subregions. This can be explained by selective far-UV photodissociation of C¹⁸O. By comparing our sample with other clouds in the plot of X13CO^/XC^18O against ¹³CO column density, we find that type I clouds are probably at an earlier evolutionary stage than type II clouds.