Edward M. Murphy

Overview of the Far Ultraviolet Spectroscopic Explorer Mission

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Angstrom, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al :LiF coatings for optimum reflectivity between approximately 1000 and 1187 Angstrom, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Angstrom. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H-2 and HD.

On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer Satellite

The launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include the initial coalignment, focusing, and characterization of the four instrument channels and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of these data and prelaunch laboratory measurements.

The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate-Velocity Arch (IV Arch) in the direction of the quasar PG 1259+593 (l = 1206, b = +581). Absorption lines from C II, N I, N II, O I, Al II, Si II, P II, S II, Ar I, Fe II, and Fe III are used to study the atomic abundances in these two halo clouds at VLSR ~ -130 km s-1 (Complex C) and -55 km s-1 (IV Arch). The O I/H I ratio provides the best measure of the overall metallicity in the diffuse interstellar medium because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 times solar, consistent with the idea that Complex C represents the infall of low-metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 times solar, which indicates a Galactic origin. We report the detection of an intermediate-velocity absorption component at +60 km s-1 that is not seen in H I 21 cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate and high-velocity gas in the Milky Way halo.

High-resolution FUV spectroscopy of the terrestrial day airglow with the Far Ultraviolet Spectroscopic Explorer

During orbital verification the Far Ultraviolet Spectroscopic Explorer obtained spectra of the terrestrial day airglow between 905 and 1184 A from an altitude of 766 km. The spectrographs have three apertures that can simultaneously record the atmospheric emissions with limiting instrumental spectral resolutions of approximately 0.4, 0.05, and 0.03 A. Seven orbits were obtained of observations of the sunlit Earth and disclose a wealth of emissions resulting from the electron impact excitation of N2 in addition to emissions of O I, N I, and N II produced by both photoelectron impact and by photodestructive excitation and ionization of thermospheric O and N2 by extreme ultraviolet solar radiation. The argon resonance transitions are unambiguously identified as are previously unreported transitions between highly excited energy levels of O+. These spectra have the highest spectral resolution and sensitivity in this spectral range to date and will provide valuable input to the interpretation of lower resolution spectra from current and future Earth remote sensing missions.

Highly Ionized High-Velocity Clouds: Intergalactic Gas in the Local Group or Distant Gas in the Galactic Halo?*

In the course of our studies of the gaseous halo surrounding the Milky Way, we have recently identified several high-velocity clouds (HVCs; VLSR -1 are favored if the clouds are located in the distant Galactic halo since the cloud sizes scale inversely with metallicity. We provide a summary of the HVCs detected in absorption at intermediate resolution with the GHRS and the IUE satellite and find that C IV HVCs are detected along three of 10 extragalactic sight lines down to a level of logN(C IV)≈13.3 (3σ).

Far Ultraviolet Spectroscopic Explorer Observations of O VI Absorption in the Galactic Halo

Far-ultraviolet spectra of 11 active galactic nuclei observed by Far Ultraviolet Spectroscopic Explorer (FUSE) are analyzed to obtain measures of O VI λ1031.93 absorption occurring over very long paths through Milky Way halo gas. Strong O VI absorption is detected along 10 of 11 sight lines. Values of log[N(O VI) sin |b|] range from 13.80 to 14.64, with a median value of 14.21. The observations reveal the existence of a widespread but irregular distribution of O VI in the Milky Way halo. Combined with estimates of the O VI midplane density, n0 = 2 × 10-8 cm-3, from the Copernicus satellite, the FUSE observations imply an O VI exponential scale height of 2.7 ± 0.4 kpc. We find that N(C IV)/N(O VI) ranges from ~0.15 in the disk to ~0.6 along four extragalactic sight lines. The changing ionization state of the gas from the disk to the halo is consistent with a systematic decrease in the scale heights of Si IV, C IV, N V, to O VI from ~5.1 to ~2.7 kpc. While conductive heating models can account for the highly ionized atoms at low |z|, a combination of models (and processes) appears to be required to explain the highly ionized atoms found in the halo. The greater scale heights of Si IV and C IV compared to O VI suggests that some of the Si IV and C IV in the halo is produced in turbulent mixing layers or by photoionization by hot halo stars or the extragalactic background.

Far Ultraviolet Spectroscopic Explore Observations of O VI in High-Velocity Clouds

We have used moderate-resolution (FWHM ≈ 25 km s-1) spectra of active galactic nuclei and QSOs observed by the Far Ultraviolet Spectroscopic Explorer to make the first definitive measurements of absorption by hot gas in high-velocity clouds (HVCs) at large distances from the Galactic plane. Seven of the 11 sight lines studied exhibit high-velocity ( > 100 km s-1) O VI λ1031.93 absorption with log N(O ) ≈ 13.79-14.62. High-velocity O VI absorption is detected in the distant gas of H I HVC complex C, the Magellanic Stream, several HVCs believed to be in the Local Group, and the outer Galaxy. The fraction of O VI in HVCs along the seven sight lines containing high-velocity O VI averages ~30%, with a full range of ~10%-70%. The O VI detections imply that hot (T ~ 3 × 105 K), collisionally ionized gas is an important constituent of the HVCs since O VI is difficult to produce by photoionization unless the path lengths over which the absorption occurs are very large (>100 kpc). The association of O VI with H I HVCs in many cases suggests that the O VI may be produced at interfaces or mixing layers between the H I clouds and hot, low-density gas in the Galactic corona or Local Group. Alternatively, the O VI may originate within cooling regions of hot gas clouds as they are accreted onto the Galaxy.

Far Ultraviolet Spectroscopic Explorer Spectroscopy of High-Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the sight line toward the Seyfert 1 galaxy Markarian 876, which passes through high-velocity cloud (HVC) complex C. This sight line demonstrates the ability of FUSE to measure ionic absorption lines in Galactic HVCs. High-velocity absorption is clearly seen in both members of the O VI doublet. This is the first detection of O VI in a neutral hydrogen HVC. One component of HVC complex C is resolved in multiple Fe II lines from which we derive N(Fe II)/N(H I) = 0.48 (Fe/H)☉. This value of N(Fe II)/N(H I) implies that the metallicity of complex C along this sight line may be higher than that along the Mrk 290 sight line (0.1 solar) found by Wakker et al. On the other hand, if the metallicity of complex C is also 0.1 solar along this line of sight, the observed value of N(Fe II)/N(H I) suggests there may be a significant amount of H+ along the line of sight. In any case, little, if any, iron can be depleted into dust grains if the intrinsic metallicity of complex C is subsolar. Absorption from complex C is also seen in C II, N I, and N II, and upper limits based on nondetections can be determined for Ar I, P II, and Fe III. Although molecular hydrogen in the Milky Way is obvious in the FUSE data, no H2 absorption is seen in the HVC to a limit N(H2) < 2.0 × 1014 cm-2. Future FUSE observations of extragalactic objects behind Galactic HVCs will allow us to better constrain models of HVC origins.

Prosaic Explanation for the Anomalous Accelerations Seen in Distant Spacecraft

A Comment on the Letter by John D. Anderson et al., Phys. Rev. Lett. 81, 2858 (1998). The authors of the Letter offer a Reply.

On-orbit performance of the Far Ultraviolet Spectroscopic Explorer (FUSE)

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is now making high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstroms). Its high effective area, low background, and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. In this paper, we describe the on- orbit performance of the FUSE satellite during its first nine months of operation, including measurements of sensitivity and resolution.