Average mass fraction of residual carbon in the degenerate core, as a function of initial mass. For models that undergo carbon burning, the mass fractions were calculated shortly before the Urca cooling stage, that is when the density reached a value of log₁₀(ρ_c/g cm⁻³) ≈ 9.0. Models that ignite residual carbon are represented with filled data points. All SERIES II models assume Z = 0.02, f_OV = 0.0, and only vary in terms of the wind scaling factor η.