Opacity for the aggregate populations used in this paper (grey area). The magenta line shows the opacity of the dust model “A” of Weingartner & Draine (2001). We also display the opacity of the bare aggregates (black line) and of the aggregates covered by a thin (black dashed line) or thick (black dotted line) ice mantle described in Ossenkopf & Henning (1994). The red lines show the aggregate opacities computed by Ormel et al. (2011) without (solid line, their model sil, gra) and with an ice mantle (dashed line, their model ic-sil, ic-gra). The numbers on the left side of the figure give the near- to far-IR opacity ratio for these dust models.