Time evolution of the metallicity of accreted gas. The blue line indicates our fiducial Z_accretion model, where it increases from Z_proto at M_⋆ = M₁ to Z_acc, max at M₂ followed by a metal-poor accretion phase. The black line indicates the simplified model (model M2var; see Fig. A.4), which has the same M₂ and the total amount of accreted metals as the blue line. If this M₂ value is lower than 0.96 M_⊙ (which approximately corresponds to when the radiative core develops), then these two models lead to the same Z_center.