On the systematics of line ratios along the helium isoelectronic sequence

Temperature and change dependence of the line ratios involving the forbidden, intercombination, and the resonance lines is investigated for helium-like ions of C, O, Ne, Si, S, Ca, and Fe. Systematic variations are found and should facilitate the interpretation of observed data. Calculations are carried out using more accurate atomic data than earlier works resulting in significant differences. A few recent observations from laboratory and astrophysical plasmas are discussed.