The opacity in the 9.7 μm silicate feature vs. near-IR color excess for the four aggregate models presented in Fig. 6. Each model is identified by a color/line style as indicated in the legend. The initial (t = 0.03 Myr) value of τ_sil and E(J − K) per unit column density is indicated by an empty square, the final value (t = 10 Myr) by an empty circle, and intermediate times are indicated by black dots.