Evolution with time of the fractional mass of the central radiative zone (top panel) and of the internal opacities (bottom), for a 1 M_⊙ star, assuming a heat injection parameter ξ = 0 and deuterium mass fraction X_D = 28 ppm. The solid and dashed lines indicate values at the base of the surface CZ and at the center, respectively. Three cases are considered: a standard case (blue line), one in which the diffusion coefficient of convective mixing is artificially increased (purple) or decreased (cyan) by a factor of 100. The gray area in the bottom panel indicates the range of values of the critical opacity at the base of the CZ in the standard case (κ_crit ~ 10 cm²g⁻¹). The triangles indicate the time at which the radiative zone appears at the center and starts growing.