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We present detailed calculations for the energy deposited by photons with energies between 10 eV and 1 MeV in spherical, uncharged interstellar dust particles. At low energies, when the wavelength is larger than the grain radius, we calculate the interaction probabilities using Mie scattering theory; at higher energies we uuse an atomic mean free path approach. The interaction of the photons with the solid produces primary photoelectrons and secondary Auger electrons, which deposit a fraction (if not all) of their kinetic energy in the solid. Our calculations are distinguished from previous efforts by the detailed treatment of the secondary Auger electrons.
Results are shown for grain sizes ranging from r~=~50\AA to 1~$\mu$m, for both graphitic and silicate (MgSiFe)O$_4$ grains. The results show clearly the effectiveness of different processes in heating the grain, and there is excellent agreement at the boundary between the Mie scattering theory calculations and mean free path method. The addition of Auger electrons substantially changes the energy deposited near atomic shell boundaries, since they are monoenergetic. We also show the photoelectric energy yield, the amount of energy leaving the dust in the ejected electrons, for each of our grains. These results should be useful in modelling early supernova remnants, AGNs, and other regions where energetic photons interact with dust grains.
This research was supported by NASA RTOP No. 188-44-53-05
