Plot of the relative absorption observed in the blue wing of the Lyman-α stellar line (blue histogram) for the transit of September 7, 2011. The dashed line shows the model with radiation pressure only; in this case, there is no absorption at radial velocities below ~−120 km s^-1, resulting in a large χ² of 22.8. If a stellar wind and charge exchange is considered, hydrogen atoms can be accelerated to the higher observed velocities. The model with an escape rate of 10⁹ g s^-1 (solid line) gives a χ² of 13.0 for 17 degrees of freedom.