The ORFEUS II Echelle Spectrum of HD 93521: A reference for interstellar molecular hydrogen

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. The spectrum of HD 93521 was obtained during this mission with a total integration time of 1740 s. This spectrum shows numerous sharp interstellar absorption lines. We identified 198 lines of molecular hydrogen including at least 7 lines with a high velocity component. Also most of the 67 identified interstellar metal lines are visible with a high velocity component. We present plots of the complete ORFEUS Echelle spectrum together with tables of all identified interstellar absorption lines including all 14 detectable HI lines. In addition several identified stellar lines, partially with narrow absorption components, and stellar wind lines are given in a separate table.Comment: 18 pages, 33 figure

The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the in-flight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than lambda/(Delta lambda) = 10000, where Delta lambda is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm^2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than +/- 0.005 nm.Comment: 8 pages, 8 figure

A FUSE Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo

Far Ultraviolet Spectroscopic Explorer (FUSE) data is used to investigate the molecular hydrogen (H_2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption towards 56 (mostly extragalactic) background sources to study H_2 absorption in the Lyman- and Werner bands in 61 IVC components at H I column densities >10^19 cm^-2. For data with good S/N (~9 per resolution element and higher), H_2 in IVC gas is convincingly detected in 14 cases at column densities varying between ~10^14 and ~10^17 cm^-2. We find an additional 17 possible H_2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10^-6 and 10^-3, implying a dense, mostly neutral gas phase that is probably related to the Cold Neutral Medium (CNM) in these clouds. If the H_2 stays in formation-dissociation equlibrium, the CNM in these clouds can be characterized by compact (D~0.1 pc) filaments with volume densities on the order of n_H~30 cm^-3. The relatively high detection rate of H_2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.Comment: 36 pages, 11 figures; accepted for publication in Ap

HIRDES - The High-Resolution Double-Echelle Spectrograph for the World Space Observatory Ultraviolet (WSO/UV)

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102-310nm spectral band is split to feed two echelle spectrographs covering the UV range 174-310nm and the vacuum-UV range 102-176nm with high spectral resolution (R>50,000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering performance aspects, design drivers, related trade-offs (mechanical concepts, material selection etc.) and a critical functional and environmental test verification approach. The current state of other WSO/UV scientific instruments (imagers) is also described.Comment: Accepted for publication in Advances in Space Researc

The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines in the FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) at intermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With a good signal-to-noise ratio and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG0804+761 is dominated by strong absorption from local Galactic gas, we concentrate our study on absorption by molecular hydrogen and neutral neutral and ionized metals related to an intermediate-velocity cloud in the lower Galactic halo at -55 km/s, and on absorption from OVI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in 5 lines for rotational levels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30). We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar. Absorption by OVI is found at velocities as negative as -110 km/s, but no absorption from any species is found at velocities of -180 km/ where absorption from the nearby high-velocity Complex A would be expected. We suggest that the extended OVI absorption traces hot gas above the Perseus spiral arm. Finally, we find intergalactic absorption by an intervening HI Ly betax absorber at z=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102. No intervening OVI absorbers are seen in the spectrum of PG0804+761.Comment: 27 pages, 6 figures; accepted for publication in Ap

FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream

We present Far Ultraviolet Spectroscopic Explorer observations of the atomic and molecular absorption in high velocity cloud HVC 287.5+22.5+240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We detect H2, N I, N II, Si II, and Fe II absorption and set limits on the amount of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity and dust-depletion measurements made by Lu and collaborators by examining the relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived gas-phase abundances due to ionized gas in the HVC are less than 15%. The HVC has a metallicity of 0.2-0.4 solar, similar to that of the Small Magellanic Cloud. The relative abundance pattern for the elements studied resembles that of warm gas in the SMC, which supports the idea that this HVC is part of the tidally stripped Leading Arm of the Magellanic Stream. The abundance pattern implies that the HVC contains dust grains that have been processed significantly. It is likely that the grain mantles have been modified or stripped back to expose the grain cores. We have identified more than 30 lines of H2 arising in the HVC from rotational levels J = 0 to J = 3. Synthetic spectra and a curve-of-growth fit to these lines with b = 12 km/sec indicate that log N(H2) = 16.80+/-0.10 and f(H2) = 2N(H2)/[N(H I)+2N(H2)] = 0.0016. From an analysis of the H2 rotational populations, we find an absorption rate (at 1000 A) that is less than one-tenth the average value in the solar neighborhood. The presence of molecular gas in the HVC requires that either the H2 formed in situ or that molecules formed within the SMC survived tidal stripping. We favor the latter possibility because of the long formation time derived for molecules in this HVC.Comment: 28 pages (includes 6 figures). AASTeX preprint format. Accepted for publication in the February 2001 issue of the Astronomical Journa