An Assessment of the Fe <scp>xviii</scp> and Fe <scp>xix</scp> Line Ratios from the <i>Chandra</i> Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, EUV, and FUV line emission, formed at the peak of Capella's (alpha Aurigae) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer, and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than factors of two are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common usage. In particular the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 10^7 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.