Radiative transfer modeling of H₂CO lines for two positions toward the Horsehead. Two left columns: the PDR position (T_kin = 60 K, n(H₂)    =    6    ×    10⁴   cm^-3, N(o - H₂CO) = 7.2    ×    10¹²   cm^-2) and two right columns: the dense-core position (T_kin = 20 K, n(H₂)    =    10⁵   cm^-3, N(o - H₂CO) = 9.6    ×    10¹²   cm^-2). The two top rows display the ortho lines, for which we varied the column density around the best match (red curve) by a factor of 1.5 (blue curve) and 1/1.5 (green curve). The two bottom rows display the para lines, for which we kept the column density of the best match for o - H₂CO (red curves) constant and varied the ortho-to-para ratio of H₂CO: o/p = 1.5 (dashed blue), o/p = 2 (dashed red) and o/p = 3 (dashed green).