Predicted P(J+2)/R(J) ≡ f_J ratios for the ¹²CO v = 1−0 band. The optically thin limit is represented by the gray crosses (Sect. 4.1.1). The black diamonds show the optically thick limit for three rotational temperatures (15, 25, and 35 K; Sect. 4.1.2). The dependence on N(CO)/σ_v (left panel), T_kin (middle panel), and (right panel) of the single component NLTE models is indicated by the colored (blue to orange/red) empty squares. Only one parameter is varied in each panel (the colored numbers indicate the value of the varied parameter). The remaining two parameters are constant and equal to the following reference values: N(CO)/σ_v = 10¹⁷⁷ cm⁻²/(km s⁻¹), T_kin = 25 K, and (see Sect. 4.1.3). The red filled circles in the left panel are the observed ratios in region N2. The red empty circles in the right panel represent the LTE limit for the reference values of T_kin and N(CO)/σ_v. The points are connected by lines to guide the eye.