Identification of FeII emission lines in FUSE stellar spectra

We identify two complexes of Fe II emission lines in far-ultraviolet spectra of the stars alpha TrA and HD 104237. Using spectra from both the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST), we show that these emission lines, which represent the majority of previously unidentified emission features in cool star spectra between 912 and 1180 Angstrom, are fluorescent decays in Fe II following excitation by H Ly alpha. Specifically, following photoexcitation from the third term (4s a D-4) of Fe II, subsequent decays are observed to the two lowest terms (4s a D-6 and 3d(7) a F-4) which are observed near 1100 and 1135 Angstrom, respectively. Decays to higher terms, and hence longer wavelengths, also are clearly seen in the STIS spectra. Differences in the fluorescent Fe II spectra of alpha TrA and HD 104237 are tentatively identified as resulting from differences in the intrinsic width of the density-weighted H Ly alpha radiation fields. The additional Fe II lines observed in alpha TrA result from a broadened H Ly alpha profile. Two features near 1060 Angstrom appear to be fluorescent lines of Cr II, also excited by H Ly alpha.