RADEX modelled excitation temperature 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⁺ (1–0) 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. The inset zooms in on 10⁻⁴ < x_e < 10⁻² with the vertical dashed purple line indicating x_e = 1.5 × 10⁻⁴ which corresponds to the value of x_e used in our calculations.