Density (top panel) and composition (bottom panel) profiles of our 15 M_⊙ model at the onset of core collapse. The model corresponds to a BSG star with a surface radius of 44.8 R_⊙. The adopted environment metallicity is 10⁻⁷. The bulk of the composition is H and He rich, and the metal-rich part is limited to the He-core below a lagrangian mass of 4 M_⊙ (see Sect. 3.1 for discussion).