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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

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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.