X-Ray Emission from Dust in Hot Plasmas

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Abstract
Energetic (few keV) thermal electrons easily penetrate interstellar dust grains immersed in hot plasmas. These electrons create K-shell vacancies in atoms locked in dust grains through the K-shell collisional ionization process. Heavy elements such as Fe and Ca produce fluorescent Kα emission in the subsequent decay of the inner shell vacancy. We predict that this emission from Fe atoms in dust grains should be present in X-ray spectra emitted by dusty plasma, in addition to Kα emission from highly ionized Fe ions. This affects the strength and shape of the Fe Kα complex. We also predict a low ratio of L-shell/Kα emission for dust grains, because of low fluorescent yields for L-shell transitions. These effects should be most pronounced in young supernova remnants (SNRs). We discuss observational evidence for the fluorescent Fe Kα emission from dust in X-ray spectra of Tycho's, Kepler's, and Cassiopeia A SNRs, the three youngest remnants in our Galaxy.