Weingartner & Draine (2001) and Li & Draine (2001) have developed a carbonaceous-silicate grain model which successfully reproduces observed interstellar extinction, scattering, and infrared emission. The model consists of a mixture of carbonaceous grains and silicate grains, each with a broad size distribution extending from a = 3.5 Angstrom ultrasmall grains (containing tens of C atoms) to micron-sized particles. The size distribution includes two log-normal distributions, with characteristic radii a_0=3.5 Angstrom and 30 Angstrom, and width parameters sigma=0.4 (see Weingartner & Draine 2001, eq. 2,3), plus two modified-power-law size distributions with smooth cutoffs at large sizes (see Weingartner & Draine 2001, eq. 4-6)
The smallest carbonaceous grains have the optical and physical properties of PAH (polycyclic aromatic hydrocarbon) molecules. The larger (a > 100 Angstrom) carbonaceous grains are assumed to have the optical properties of graphite spheres. The silicate grains have optical properties appropriate to amorphous olivine, with the far-infrared opacity adjusted to reproduce the observed far-infrared and submm emission from interstellar dust (see Li & Draine 2001).
Draine (2003a) discussed the available evidence for dust extinction per unit H column in regions with different extinction curves. Based on this discussion, we have renormalized the size distributions of Weingartner & Draine (2001). For R_V=3.1, we have reduced the grain abundance per H by a factor 0.93 For R_V=4.0, we have taken the "A" model of Weingartner and Draine (2001) and increased the grain abundance per H by a factor 0.93*1.27=1.18. For R_V=5.5, we have taken the R_V=5.5 "A" model of Weingartner & Draine (2001) and increased the grain abundance per H by a factor 0.93*1.52=1.42
Dielectric functions were recently revised by Draine (2003b,c), including realistic structure at X-ray absorption edges of C, O, Mg, Si, and Fe.
As described in Draine (2003a) and Draine (2003b), extinction, absorption, albedo, <cos(theta)>, and <cos^2(theta)> have been calculated for wavelengths from 1 cm (30 GHz) to 1 Angstrom (12.4 keV), for selected mixtures of carbonaceous grains and amorphous silicate grains:
These older results continue to be available, based on the size distributions and dielectric functions as described in Weingartner & Draine (2001):
for wavelengths running from 0.01 micron to 1 cm. Here b_C = total C/H in the two log-normal size distributions of ultrasmall carbonaceous grains.
N.B.: The tabulated opacity K_abs is the ratio (dust absorption cross section)/ (dust mass). To obtain the ratio (dust absorption cross section)/(total mass), the tabulated K_abs should be divided by (1+M_gas/M_dust). For the Milky Way dust mixtures, (1+M_gas/M_dust) = 125.
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