Central and surface properties of 15 M_⊙ models. Left panel: mass of the convective core in solar mass as a function of the central mass fraction of hydrogen X_c in different 15 M_⊙ models. Right panel: evolution of the surface abundance ratio N/H (normalized to its initial value and in logarithmic scale) as a function of log g_surf for the same models as presented in the left panel. In both panels, the letters have the same meaning as in Fig. 2.