A physical grain model of PAHs, carbonaceous grains, and amorphous silicate grains has been constructed to reproduce observations of interstellar extinction and infrared emission (Weingartner & Draine 2001a; Li & Draine 2001a). The material dielectric functions and heat capacities, and absorption cross sections for the PAHs, are consistent with laboratory data and physics; once the size distribution is specified, the properties of the grain model (scattering, extinction, infrared emission) can be calculated.
The grain size distribution for average diffuse clouds (RV 3.1 in our region of the Milky Way is shown in Figure 7. The mass distribution must peak near ~ 0.3 µm in order to reproduce the observed extinction near visual wavelengths. The peak in the PAH distribution near .0005 µm is required for single-photon heating to reproduce the observed 3-12 µm emission, but the secondary peak near .005 µm in Figure 7 may be an artifact of the fitting procedure.
Figure 7. Size distributions for dust model of Weingartner & Draine (2001a).
Milky Way extinction curves with different RV values, or extinction curves for the LMC and SMC, can be reproduced by varying the size distribution. The grain model appears to be consistent with the observed scattering properties of interstellar dust in the optical and ultraviolet (Draine 2003b) and at X-ray energies (Draine 2003c).
The grain model also reproduces the observed infrared emission in the diffuse ISM of the Milky Way (Li & Draine 2001a), reflection nebulae (Li & Draine 2002b), and the SMC Bar (Li & Draine 2002c).