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Session 74 - Supernova Remnants.
Display session, Thursday, June 13
Great Hall,
Refractory elements in the interstellar medium are almost always heavily depleted from gas onto dust grains. They remain locked in dust grains for a long time after the passage of a supernova blast wave, as demonstrated by observations of infrared emission from supernova remnants (SNRs). Dust grains contain elements such as Mg, Si, Ca, and Fe which, while in gaseous phase, produce strong X-ray lines frequently seen in X-ray spectra of SNRs. In general, the depletion of these elements onto dust grains reduces the strength of their X-ray lines, in the amount proportional to their depletion. However, energetic thermal electrons in young SNRs can penetrate the dust grains, and eject electrons from the innermost shell of all elements. Heavy elements such as Ca and Fe will produce fluorescent K\alpha emission in the subsequent decay of the inner shell vacancy. We predict that this emission from ``cold'' Fe and Ca atoms in dust grains should be present in X-ray spectra of young SNRs, in addition to K\alpha emission from highly ionized Fe and Ca ions present in hot gas. This will affect the strength and shape of K\alpha complexes of these elements. We also predict a low ratio of L-shell/K\alpha emission for dust grains, because of low fluorescent yields for L-shell transitions. We discuss consequences of the dust presence on interpretation of X-ray spectra obtained by the Advanced Satellite for Cosmology and Astrophysics (ASCA) in Tycho, Kepler, and Cassiopeia A, the three youngest SNRs in our Galaxy.
