Theoretical absorption depth of Mg i, as calculated using numerical simulations of the escape of magnesium atoms accelerated by stellar radiation pressure. Upper panel: AD as a function of the radial velocity for various ionization cut-off radii (in R_P units). We find that a cut-off of 7.5 R_P provides an AD profile that matches the velocity range of the observed absorption on the blue edge (indicated by the leftward vertical line); the other velocity limit shown is around 10 km s^-1 (the other vertical line), which is independent of the radiation pressure model calculation, since it is related to the assumed blow-off velocity; this profile is obtained for a magnesium mass loss rate of 3 × 10⁷ g s^-1. Lower panel: same as above for an ionization cut-off radius of 7.5 R_P and two different Mg i escape rates. This shows that the two main parameters of the model (escape rate and cut-off radius) can be constrained independently through the measurement of the absorption depth and the velocity range of the gas.