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Infrared (IR) emission spectra are calculated for dust composed of mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented, that are consistent with observed emission spectra, including those newly obtained by Spitzer Space Telescope. We find a size distribution for the PAHs that results in emission band ratios consistent with observed spectra of the Milky Way and other galaxies. Emission spectra are presented for various intensities of the illuminating starlight. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7. 7um emission feature is produced mainly by PAH particles containing Umin. We present graphical procedures using IRAC and MIPS photometry to estimate qpah, Umin, and gamma, the fraction fPDR of the dust luminosity coming from photodissociation regions with U>100, and the total dust mass Mdust.
Draine et al. (Tue,) studied this question.