H/He gradient dX/dQ (cf. Fig. 1 in Sect. 2) as a function of stellar radius for 32 M_⊙ sequences computed with various assumptions on internal mixing. Thick solid lines indicate core hydrogen burning and thin solid lines indicate core helium burning. The short-lived in-between phase is displayed with a dashed line. In the top panel, α_ov = 0.33 is adopted in all models while α_sc is varied. In the bottom panel, all models are computed with α_sc = 1 while α_ov is varied.