Photospheric velocity used in the reference model (points). The two solid curves show the expected evolution of the photospheric velocity calculated for the reference density structure adopting a constant opacity of 0.03 or 0.6 cm² g⁻¹. In TARDIS spectral synthesis simulations, the input photospheric velocity is varied as a parameter within the range between the two lines shown here (see Sect. 4.6).