Measured flux of the Ly-α emission line as a function of radial velocity in the β Pic reference frame. For sake of clarity each data point has been created by calculating the average of three individual measurements. The lines in green, red and blue correspond to values of log (N_H/ 1 cm²) of the β Pic absorption component equal to 18.0, 18.6 and 19.0 respectively, whilst keeping all other parameters constant. Fits generated with log (N_H/ 1 cm²) different from the best fit at 18.6 produce significantly worse fits.