Fig. 4.

Example of ortho- and para-${\text{H}}_{\text{2}}^{\text{13}}\text{CO}$ lines (black histogram spectra). A non-LTE LVG model (T_k ≃ 200 K, n(H₂) ≃ 1 × 10⁶ cm⁻³, $N{(\text{o}-\text{H}}_{\text{2}}^{\text{13}}\text{CO})=7.3\times {\text{10}}^{\text{11}}{\text{cm}}^{-2},$ and $N{(\text{p}-\text{H}}_{\text{2}}^{\text{13}}\text{CO})=2.3\times {\text{10}}^{\text{11}}{\text{cm}}^{-2})$ is shown overlaid in red (see Sect. 5.2).