Best-fit DL07 emissivity index β_DL07 plotted against U_min (in the left panel) and ⟨ U ⟩ (in the right). Only those MBBV DL07 fits with ≤ 2 are considered in the plot. As before, points are coded by oxygen abundance. The curve in the left panel is the second-order polynomial best-fit of DL07 MBBV β to log ₁₀(U_min): β_DL07 = 1.72 + 0.58 log (U_min) − 0.20 log ²(U_min). The right panel shows the analogous curve for log ₁₀(⟨ U ⟩): β_DL07 = 1.56 + 0.77 log ( ⟨ U ⟩ ) − 0.23 log ²( ⟨ U ⟩).