Comparison of the simulated line profiles of [¹²C II] and [¹³C II] (top) and the line ratio (bottom) equivalent to the plots in Figs. 3b–6b. An intrinsic abundance ratio α⁺ = 67 is assumed. The underlying optical depth profile is Gaussian with a width Δv = 12.5 km s⁻¹ and a peak value of τ₁₂ = 3. Similar to the observations of Mon R2, noise with σ = 0.25 K was added and the line ratio is only measured above 1.5σ.