Convectively unstable layers in the outer layers of Vega’s model as shown by the squared Brunt-Väisälä frequencyN². Left: a general view showing the regions where N² < 0. These are limited by the solid lines. In the top panel, there are two convective layers, and the shallowest one reaches the surface of the star. The dashed line marks the minimum of N². The layers associated with helium ionization region clearly appear. Left: closer view of the surface layer. The grey scale represents the values of N² in squared radians per second.