Behaviour of g_rad for a single atomic line with respect to the relative concentration of the considered ion (see text). The dashed line is for a Lorentzian profile (α = −0.5), the dash-dotted line is the approximation for a Gaussian profile (α = −1.0), and the continuous line mimics the case of a Voigt profile (α = −0.7). Panel (a): γ_rad (the dimensionless normalized g_rad) vs X (the logarithm of the dimensionless normalized concentration). Panel (b): Logarithmic derivative of γ_rad. The minimum value is close to the exponent α. The increase in the derivative after the minimum depends on the value assumed for the contribution of photoionisation to g_rad; here it is set to 10⁻³ (see details in Alecian & LeBlanc 2000).