(a) Relation between X(¹³CO)/X(C¹⁸O) and N(¹³CO). The red, blue, green, and gray dots correspond to pixels in the W5-E1, W5-M, W5-W1 and W5-W2 regions, respectively. The gray dashed rectangles delineate the principal distribution areas within the Planck cold clumps (Wu et al. 2012), and the Orion molecular clouds (Shimajiri et al. 2014), corresponding to type I and typeII molecular clouds (Wang et al. 2019), respectively. The green line indicates the terrestrial ratio X(¹³CO)/X(C¹⁸O) = 5.5 (Wilson & Matteucci 1992). (b) Relation between X(¹³CO)/X(C¹⁸O) and T_ex. Note that only those pixels with both detectable (larger than 3σ) ¹³CO and C¹⁸O emissions are plotted.