An Assessment of the Fe xviii and Fe xix Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, EUV, and FUV line emission, formedat the peak of Capella's (alpha Aurigae) emission measure distribution andubiquitous in spectra of many cool stars and galaxies, provide a uniqueopportunity to test the robustness of Fe XVIII and Fe XIX spectral models. TheAstrophysical Plasma Emission Code (APEC) is used to identify over 35 linesfrom these two ions alone, and to compare model predictions with spectraobtained with the Chandra Low Energy Transmission Grating and High EnergyTransmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer,and the Extreme Ultraviolet Explorer. Some flux discrepancies larger thanfactors of two are found between observations of Fe XVIII and Fe XIX lines andpredictions by APEC and other models in common usage. In particular the X-rayresonance lines for both ions are stronger than predicted by all modelsrelative to the EUV resonance lines. The multiwavelength observationsdemonstrate the importance of including dielectronic recombination and protonimpact excitation, and of using accurate wavelengths in spectral codes. Theseions provide important diagnostic tools for 10^7 K plasmas currently observedwith Chandra, XMM-Newton, and FUSE.Comment: 16 pages including 1 table and 2 figure