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 best-fit parameters (solid line: escape rate Ṁ = 10⁹ g s^-1 and ionizing flux F_ion = 3  F_⊙; dashed line: escape rate Ṁ = 10¹⁰ g s^-1 and ionizing flux F_ion = 4  F_⊙). The simulated planetary disk is the source for an absorption depth of ~1.5% at all wavelengths. A low escape rate results in a slightly decreased absorption depth at low velocity (solid line), while a high ionizing flux limits the number of atoms accelerated to high velocities (dashed line).