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K.C. Roth, B-G Andersson (Johns Hopkins U), E.B. Jenkins, D.V. Bowen (Princeton U), G. Sonneborn (NASA/GSFC), FUSE Science Team
We present far UV absorption line spectra of low-ionization species in the ISM, obtained during early operations of the FUSE satellite. Targeted species include C I, Cl I, O I, N I and Fe II. O I and N I in particular have features in the far UV with transition strengths that allow for accurate column densities to be determined in a greater variety of sightlines than in other wavelength bands, for example that covered by STIS onboard HST. Because O I is observed to accurately trace the abundance of H I, measurements of O I column density may be helpful in deriving relative abundance estimates due to the saturated nature of H I absorption features and the difficulties in deriving H I column densities in individual absorption features. Lines of low-ionization atomic species, together with those of molecular H2, reveal information concerning the physical conditions and depletion due to dust within the absorbing material. Since these ions primarily exist in cool, dense regions, a comparison of the velocity structures in high and low ions is important for understanding the relationship between cool and hot gas in the ISM. In addition, C I and Fe II are among those ions commonly studied at high redshift in damped Ly\alpha absorbers. FUSE sightlines toward extragalactic objects provide important fiducial measurements against which such QSO observations can be compared.
Observations of low-ionization species, particularly C I and Cl I, also serve as useful FUSE calibrators. Because these lines originate in cool, dense material, they are likely to show narrow line shapes. Also, the exact absorption profiles and absolute velocities have been previously measured for many sightlines with STIS onboard HST. We have planned observations of several such stars which will allow us to determine the shape of the line spread function, intrinsic resolution, and absolute wavelength scale of the four FUSE spectrographs.
This work is supported by NASA contract NAS5-32985.
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