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New X-ray observatories (Chandra and XMM-Newton) are providing a wealth ofhigh-resolution X-ray spectra in which hydrogen- and helium-like ions areusually strong features. We present results from a new collisional-radiativeplasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomicdata in the companion Astrophysical Plasma Emission Database (APED) tocalculate spectral models for hot plasmas. APED contains the requisite atomicdata such as collisional and radiative rates, recombination cross sections,dielectronic recombination rates, and satellite line wavelengths. We comparethe APEC results to other plasma codes for hydrogen- and helium-likediagnostics, and test the sensitivity of our results to the number of levelsincluded in the models. We find that dielectronic recombination withhydrogen-like ions into high (n=6-10) principal quantum numbers affects somehelium-like line ratios from low-lying (n=2) transitions.Comment: 5 pages, 6 figures, accepted by ApJ Letter

Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions