RADEX modelled excitation temperatures (left) and optical depths (right) as a function of electron density, for fixed values of the total gas density at 35 (red), 100 (orange) and 1000 cm⁻³ (dark blue) for the ArH⁺ (2–1) transition. Furthermore, each model is run for fixed gas temperatures at 30 (solid), 300 (dashed) and 3000 K (dotted), respectively. The horizontal dashed grey line marks an excitation temperature of 2.73 K (in the left-hand panel) while the vertical dashed purple line indicates, x_e = 1.5 × 10⁻⁴.