Abstract

Abstract We present analyses of the 13 CO abundance and the abundance ratio of molecular clouds by collecting 12 CO, 13 CO, and C 18 O (1–0) transitions toward the Galactic anti-center in the ranges and . The area contains eight clouds, which can be divided into two types according to . Type I clouds, including the chain of Lynds dark clouds, West Front, Gemini OB1 giant molecular cloud (GGMC) 1, and Horn, have lower (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 (mean value >10) and higher column density with relatively higher temperature. For all clouds, increases from at the edge to in the interior, which is probably due to an opacity effect of 12 CO. In GGMC 2–4, the mean 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 C 18 O. By comparing our sample with other clouds in the plot of against 13 CO column density, we find that type I clouds are probably at an earlier evolutionary stage than type II clouds.