We report the results of a FUSE study of high-velocity O vi absorption along complete sight lines through the Galactic halo in directions toward 100 extragalactic objects and two halo stars. The high-velocity O vi traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflowing material from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 84 high-velocity O vi features at !3 confidence at velocities of À500 < v LSR < þ500 km s À1 . The 84 O vi features have velocity centroids ranging from À372d" v v LSR d À 90 km s À1 to þ93d" v v LSR d þ 385 km s À1 , line widths b $ 16 72 km s À1 with an average of hbi ¼ 40 AE 13 km s À1 , and an average O vi column density hlog Ni ¼ 13:95 AE 0:34 with a median value of 13.97. Values of b greater than the 17.6 km s À1 thermal width expected for O vi at T $ 3 Â 10 5 K indicate that additional nonthermal broadening mechanisms are common. The O vi 1031.926 absorption is detected at !3 confidence along 59 of the 102 sight lines surveyed. The high-velocity O vi detections indicate that $60% of the sky (and perhaps as much as $85%, depending on data quality considerations) is covered by high-velocity H + associated with the O vi. We find that NðH þ Þe10 18 cm À2 if the high-velocity hot gas has a metallicity similar to that of the Magellanic Stream; this detection rate is considerably higher than that of high-velocity warm H i traced through its 21 cm emission at a comparable column density level. Some of the high-velocity O vi is associated with known H i structures (the Magellanic Stream, Complex A, Complex C, the Outer Spiral Arm, and several discrete H i HVCs). Some of the high-velocity O vi features have no counterpart in H i 21 cm emission, including discrete absorption features and positive velocity absorption wings extending from $100 to $300 km s À1 that blend with lower velocity absorption produced by the Galactic thick disk/halo. The discrete features may typify clouds located in the Local Group, while the O vi absorption wings may be tidal debris or material expelled from the Galactic disk. Most of the O vi features have velocities incompatible with those of the Galactic halo, even if the halo has decoupled from the underlying Galactic disk. The reduction in the dispersion about the mean of the high-velocity O vi centroids when the velocities are converted from the LSR to the GSR and LGSR reference frames is necessary (but not conclusive) evidence that some of the clouds are located outside the Galaxy. Most of the O vi cannot be produced by photoionization, even if the gas is irradiated by extragalactic ultraviolet background radiation. Several observational quantities indicate that collisions in hot gas are the primary ionization mechanism responsible for the production of the O vi. These include the ratios of O vi column densities to those of other highly ionized species (C iv, N v) and the strong correlation between N(O vi) and O ...

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of

We describe a moderate-resolution FUSE survey of H 2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H 2 Lyman and Werner bands between 912 and 1120 Å. Our survey is sensitive to N(H 2 ) ≥ 10 14 cm −2 ; the highest column densities are log N(H 2 ) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H 2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions f H2 = 0.010 +0.005 −0.002 for the SMC and f H2 = 0.012 +0.006 −0.003 for the LMC, compared with f H2 = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil-beam sight lines as measures of N(H I). These results imply that the diffuse H 2 masses of the LMC and SMC are 8 × 10 6 M ⊙ and 2 × 10 6 M ⊙ , respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H 2 formation rate coefficient, R ∼ 3 × 10 −18 cm 3 s −1 , and incident radiation fields ranging from 10 -100 times the Galactic mean value. We find that these high-radiation, low-formation-rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H 2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and find that the distribution of rotational temperatures is similar to Galactic gas, with T 01 = 82 ± 21 K for clouds with N(H 2 ) ≥ 10 16.5 cm −2 . There is only a weak correlation between detected H 2 and far-infrared fluxes as determined by IRAS, perhaps due to differences in the survey techniques. We find that the surface density of H 2 probed by our pencil-beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

No abstract

We obtained high-resolution FUSE (R ∼ 20,000) and VLT (R ∼ 45,000) spectra of the quasar HE2347-4342 to study the properties of the intergalactic medium between redshifts z = 2.0 − 2.9. The high-quality optical spectrum allows us to identify approximately 850 H I absorption lines with column densities between N ∼ 5 × 10 11 and 10 18 cm −2 . The reprocessed FUSE spectrum extends the wavelength coverage of the He II absorption down to an observed wavelength -2of 920Å. Source flux is detected to restframe wavelengths as short as ∼ 237Å. Approximately 1400 He II absorption lines are identified, including 917 He II Lyα systems and some of their He II Lyβ, Lyγ, and Lyδ counterparts. The ionization structure of He II is complex, with approximately 90 absorption lines that are not detected in the hydrogen spectrum. These features may represent the effect of soft ionizing sources. The ratio η=N(He II)/N(H I) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indices of quasars. This provides evidence that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured AGN, which do not ionize helium. We find an evolution in η toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z > 2.7, the large but finite increase in the He II opacity, τ = 5 ± 1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He + ions. For line widths b He + = ξb H , we find ξ = 0.95 ± 0.12, indicating a velocity field in the intergalactic medium dominated by turbulence. At hydrogen column densities N < 3 × 10 12 cm −2 the number of forest lines shows a significant deficit relative to a power law, and becomes negligible below N = 10 11 cm −2 .

Results from the occultation of the sun by Neptune imply a temperature of 750 +/- 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 +/- 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 +/- 5 kelvins, and the surface pressure is roughly 14 microbars.

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 10(5) K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman alpha by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (>/= 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

We report new observations of the spectrum of Ganymede in the spectral range 1160È1720 made A with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space T elescope (HST ) on 1998 October 30. The observations were undertaken to locate the regions of the atomic oxygen emissions at 1304 and 1356 previously observed with the Goddard High Resolution Spectrograph on HST , that A , Hall et al. claimed indicated the presence of polar aurorae on Ganymede. The use of the 2A wide STIS slit, slightly wider than the disk diameter of Ganymede, produced objective spectra with images of the two oxygen emissions clearly separated. The O I emissions appear in both hemispheres, at latitudes above o 40 o ¡, in accordance with recent Galileo magnetometer data that indicate the presence of an intrinsic magnetic Ðeld such that Jovian magnetic Ðeld lines are linked to the surface of Ganymede only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the four contiguous orbits (5.5 hr) of observation, presumably because of the changing Jovian plasma environment at Ganymede. However, the observed longitudinal nonuniformity in the emission brightness at high latitudes, particularly in the southern hemisphere, and the lack of pronounced limb brightening near the poles are difficult to understand with current models. In addition to observed solar H I Lya reÑected from the disk, extended Lya emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb, and its brightness is consistent with the Galileo UVS measurements of Barth et al.