Compactness parameter as a function of initial mass (top) and final mass (bottom) for helium star models of different metallicities. Circles represent models with final surface abundances that correspond to Type Ib SN progenitors, whereas crosses represent models with surface abundances that correspond to Type Ic SN progenitors. The dashed horizontal line at ξ_2.5 = 0.35 divides the majority of exploding models from those that produce BHs.