a) The H₃O⁺ rotation diagram (triangles), calculated in the optically thin limit. The colored squares (red: ortho, blue: para) show the expected column density distribution for a formation temperature of T_form = 1000 K, and neglecting collisional relaxation. The two solid lines correspond to rotational temperatures of T_rot = 250 and 600 K. b) The rotational temperature of the metastable level of H₃O⁺ relative to the one as a function of the electron abundance χ(e) for two adopted values of T_gas (50 and 200 K) and four assumed values of (curves 1), 1 × 10^-10 (2), 3 × 10^-10 (3), and 1 × 10^-9 cm³ s^-1 (4).