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

New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of high-resolution X-ray spectra in which hydrogenand helium-like ions are usually strong features. We present results from a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (apec), which uses atomic data in the companion Astrophysical Plasma Emission Database (aped) to calculate spectral models for hot plasmas. aped contains the requisite atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. We compare the apec results to other plasma codes for hydrogenand helium-like diagnostics, and test the sensitivity of our results to the number of levels included in the models. We find that dielectronic recombination with hydrogen-like ions into high (n = 6 − 10) principal quantum numbers affects some helium-like line ratios from low-lying (n = 2) transitions. Subject headings: atomic data — atomic processes — plasmas — radiation mechanisms: thermal — Xrays: general