Test of the velocity gradient effect. The top panel represents a single plasma layer in a stellar wind, with a velocity gradient. The layer is at uniform temperature T, with one radiating species, assuming LTE and no scattering. The layer is modeled in IRIS with one cell between grid points z₁ and z₂, with wind velocities V₁ > 0 (the material approaches the observer) and V₂ > V₁. The red thick curve in the bottom left panel displays the monochromatic specific intensity I_ν, due to the radiation that emerges from the layer at z₂ toward the observer, as computed by IRIS. This emission spectrum is plotted versus photon energy hν (eV) or photon wavelength λ   (Å). It is obtained with a parameter ϵ_D = 0.3. Too large values of ϵ_D result in incorrect profiles, as shown by the following curves: cyan (ϵ_D = 11), green (ϵ_D = 6), orange (ϵ_D = 5), blue (ϵ_D = 3), grey (ϵ_D = 2). The bottom right panel displays the corresponding transmission spectrum. For comparison, the black thick curve portrays the spectral line that would obtain if there was no velocity gradient (V₂ = V₁). See Sect. 8.3 for detailed explanations.