Spin magnitude χ₂ of the BH formed via direct core collapse of the He star (left panel) and the corresponding total angular momentum of the progenitor star (right panel) as a function of the central helium abundance. The binary system consists of a He star with an initial mass of 20 M_⊙ in a 1.0-day orbit, and a BH of varying mass (gray solid line: 3 M_⊙; cyan solid line: 25 M_⊙; black dashed line: 40 M_⊙). The results at 0.01 Z_⊙ are nearly identical to those at 0.1 Z_⊙, and are therefore not shown in the paper.