Fe II emission lines. I. Chromospheric spectra of red giants

High quality, high-dispersion, long-wavelength IUE spectra of three well-studied giant stars, α Boo (K1 III), α Tau (K5 III) and β Gru (M5 III), are constructed using a "difference filtering" algorithm developed by Ayres. Measurements of all the emission lines seen between λλ2230 and 3100 Å are tabulated. We take special care to measure all accessible Fe II lines between the 3d⁶4s, 3d⁷ and 3d⁶4p configurations including upper limits for absent (but important) members of multiplets. The emission spectrum of Fe II is discussed in comparison with other lines (e.g., Mg II h and k) whose formation mechanisms are well-understood. We relate systematic changes in the Fe II spectrum to the different physical conditions in the three stars, and give examples of line profiles and ratios which can be used to determine conditions in the outer atmospheres of giants. We conclude that most of the Fe II emission results from collisional excitation and/or absorption of photospheric photons at optical wavelengths, but some lines are formed by fluorescence, being photoexcited by other strong chromospheric lines. Between 10% and 20% of the radiative losses of Fe II arise from 10 eV levels radiatively excited by the strong chromospheric H Lyα line. Total radiation losses in Fe II lines from red giant chromospheres are less than the losses from Mg II h and k, in sharp contrast to recent models of the solar chromosphere. We ascribe this difference mainly to radiative transfer effects instead of poorly known collision rates. The data presented here will be used in future papers to constrain atomic excitation parameters and models of the chromospheric emitting regions and stellar winds.