High-Resolution Measurements of the K alpha Spectra of Low-Ionization Species of Iron: A New Spectral Signature of Nonequilibrium Ionization Conditions in Young Supernova Remnants

We present the first systematic laboratory measurements of high-resolution Kα spectra of intermediate ions of iron, Fe X-XVII. These lines are not produced in collisional equilibrium plasmas because the relevant charge states cannot exist at the high electron temperatures required for appreciable excitation of the Kα transitions. However, they can provide excellent spectral diagnostics for nonequilibrium ionization conditions, such as the ionizing plasmas of young supernova remnants. To facilitate the line identifications, we compare our spectra with theoretical atomic calculations performed using multiconfiguration parametric potential and Dirac-Fock atomic codes. Our measurements also allow direct comparison with time-dependent ionization balance calculations for ionizing plasmas, and good agreement is found.