a) Hertzprung-Russell diagram showing the evolutionary tracks of non-rotating 32 M_⊙ (solid) and 40 M_⊙ models (dashed), with the red circles representing their endpoints. The location of the possible progenitor of iPTF13bvn is depicted by the brown region, whose size does not correspond to a formal error analysis and is merely indicative of the possible location of the progenitor. The color code refers to the mass at each timestep divided by M_ini. The endpoints of the non-rotating 30 and 50 M_⊙ models are also shown (blue circles). b) Continuum-normalized optical spectrum of the 32 M_⊙ model at the pre-SN stage, showing it is a typical WN7–8 star (G13). The strongest features are identified. c) Chemical structure of our 32 M_⊙ model at the end of the core carbon-burning stage, when the model has 10.9 M_⊙. The variations in the mass fraction of different species is shown as a function of the radius, with the surface of the star being to the right, at 9.69 R_⊙, and the center to the left at 0.0 R_⊙.