Absorption profiles during the transit of HD 209458b. The absorption signature observed in 2001 in the blue wing of the Lyman-α line is delimited by vertical black dotted lines (blue histogram; Vidal-Madjar et al. 2008), and the striped gray zone corresponds to the region contaminated by the geocoronal emission. Two absorption profiles are calculated for the same best-fit parameters (escape rate Ṁ = 10¹⁰ g s^-1; ionizing flux F_ion = 4  F_⊙), one with self-shielding (black solid line) and the other without (black dashed line). Taking into account self-shielding decreases the absorption depth at high velocities.