Fe II emission lines. II. Excitation mechanisms in cool stars

We discuss excitation mechanisms for the "resonance" transitions (between the 3d⁶4s, 3d⁷, and 3d⁶4p configurations) of Fe II observed in emission in the near-ultraviolet spectra of cool stars. Our analysis is based upon (a) emission measure analysis of previously measured lines in IUE spectra of cool giants, (b) discussion of the behavior of Fe II lines observed above the solar limb from Skylab spectra, (c) approximate radiative transfer calculations in a 59 level Fe II model atom using mean escape probabilities and a parameterization of optical radiation fields, and (d) accurate radiative transfer calculations in a smaller atomic model. The solar spectra show unambiguous evidence that electron collisions are responsible for most of the Fe II emission observed above the white-light limb. For cool giants the z ⁶D⁰ z ⁶F⁰ terms, leading to UV multiplets 1, 32, and 2, 33, 60, are also excited primarily by electron collisions. However, the z ⁶P⁰, z ⁴D⁰ z ⁴F⁰ and z ⁴P⁰ terms, leading to UV multiplets, 3-6, 34-36, and 61-64, cannot be excited solely by electron collisions or by previously identified line fluorescence processes. These terms are excited by electron excitation of metastable quartet terms below ∼4 eV, followed by photoexcitation in lines at optical wavelengths by photospheric radiation. We construct a "cool star" diagnostic diagram similar to the figure of Viotti, showing the regimes in which electron collisions and continuum photoexcitation are important in the chromospheres of cool stars. Finally, we discuss some implications of our findings.