Observed intensity ratio (lower limit) of I_ν(9 μm)/I_ν(18 μm) (black dashed line) is compared with expected intensity ratios as a function of a_min for different a_max and dust compositions. The red, blue, and gray curves indicate astronomical silicate, glassy carbon, and bulk dust emitting blackbody radiation, respectively. For astronomical silicate and glassy carbon, the solid, dashed, dash-dotted, and dotted curves indicate a_max of 1 μm, 2 μm, 5 μm, and 1 mm, respectively.