Identification of FeiiEmission Lines inFUSEStellar Spectra

Abstract

We identify two complexes of Fe II emission lines in far-ultraviolet spectra of the stars α 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 unidentifed emission features in cool star spectra between 912 and 1180 A, are fluorescent decays in Fe II following excitation by H Lyα. Specifically, following photoexcitation from the third term (4s a 4D) of Fe II, subsequent decays are observed to the two lowest terms (4s a 6D and 3d7 a 4F) which are observed near 1100 and 1135 A, 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 α TrA and HD 104237 are tentatively identified as resulting from differences in the intrinsic width of the density-weighted H Lyα radiation fields. The additional Fe II lines observed in α TrA result from a broadened H Lyα profile. Two features near 1060 A appear to be fluorescent lines of Cr II, also excited by H Lyα.