ULTRAVIOLET HIGH-EXCITATION FE-II FLUORESCENCE LINES EXCITED BY O-VI, C-IV, AND H-I RESONANCE EMISSION AS SEEN IN IUE SPECTRA

The major goal of this study was to use archival high-dispersion spectra from the International Ultraviolet Explorer (IUE) in a search for Bowen emission lines of Fe II excited by the stronger transition (lambda-1032) of the O VI resonance doublet. We also explore the possibility of using these Fe II emission lines as a diagnostic of the strength of the far-UV emission of O VI at 1032 angstrom, which lies beyond the wavelength ranges of the Hubble Space Telescope (HST) and the highly successful IUE. We searched a large sample of IUE archival SWP high-dispersion spectra of central stars of planetary nebulae, symbiotic stars, pulsating degenerate (PG 1159-type) stars, novae, supernovae, and active galactic nuclei (AGNs) that may display the high-excitation fluorescence emission lines of Fe II, recently discovered by Johansson to be excited by Bowen mechanisms driven by resonance line radiation from O VI, C IV, and H I. We have searched for these Fe II lines at lambda-lambda-1360, 1776, 1869, 1881, 1884, and 1975 in about 40 objects of the types mentioned. These lines are quite strong in symbiotic stars, particularly the "symbiotic novae" which suggests that high-excitation, high-temperature, and high electron density are essential requirements, but these Fe II lines were not detected in the extragalactic AGNs, namely NGC 1068, NGC 4151, Fairall 9, 3C 273, AKN 120, and MKN 509.