Helium-like ions as powerful X-ray plasma diagnostics

We revisited the calculations of the ratios of the Helium-like ion ``triplet'' (resonance, intercombination, and forbidden lines) for Z=6 to 14 (C V, N VI, O VII, Ne IX, Mg XI, Si XIII) in order to provide temperature, density and ionization diagnostics for the new high-resolution spectroscopic data of Chandra and XMM-Newton. Comparing to earlier computations, collisional rates are updated and the best experimental values for radiative transition probabilities are used. The influence of an external radiation field (photo-excitation) and the contribution from unresolved dielectronic satellite lines to the line ratios are discussed. Collision-dominated plasmas (e.g. stellar coronae), photo-ionized plasmas (e.g. AGNs) or transient plasmas (e.g. SNRs) are considered.