X-RAY-EMISSION FROM DUST IN HOT PLASMAS

Energetic (few keV) thermal electrons easily penetrate interstellar du st grains immersed in hot plasmas. These electrons create K-shell vaca ncies in atoms locked in dust grains through the K-shell collisional i onization process. Heavy elements such as Fe and Ca produce fluorescen t K alpha 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 K alp ha emission from highly ionized Fe ions. This affects the strength and shape of the Fe K alpha complex. We also predict a low ratio of L-she ll/K alpha emission for dust grains, because of low fluorescent yields for L-shell transitions. These effects should be most pronounced in y oung supernova remnants (SNRs). We discuss observational evidence for the fluorescent Fe K alpha emission from dust in X-ray spectra of Tych o's, Kepler's, and Cassiopeia A SNRs, the three youngest remnants in o ur Galaxy.