  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.
