Fig. 3.

Example of ortho- and para-H₂CO lines (black histogram spectra). A non-LTE LVG model (T_k ≃ 200 K, n(H₂) ≃ 1 × 10⁶ cm⁻³, N(o-H₂CO) = 4.0 × 10¹³ cm⁻², and N(p-H₂CO) = 1.8 × 10¹³ cm⁻²) is shown overlaid in red (see Sect. 5.2). The dashed lines indicate the LSR velocity (10.7 km s⁻¹) of the Orion Bar PDR.