We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright AGN at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths ∆z HI = 21.7 and ∆z OVI = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively-defined as possible. The cumulative column-density distribution of H I systems can be parametrized dN (> N )/dz = C 14 (N/10 14 cm −2 ) −(β−1) , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ (1 + z) 2.3±0.1 , and slope β(z) = 1.75 − 0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N HI . The distribution of O VI absorbers appear to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of ∆v = 50 − 300 km s −1 with no significant clustering at ∆v 1000 km s −1 . Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N HI 10 13.5 cm −2 , metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at

Using the new Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we obtained moderate-resolution, high signal/noise ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit, both orbital quadratures, and secondary eclipse. We compare transit spectra with spectra obtained at non-transit phases to identify spectral features due to the exoplanet's expanding atmosphere. We find that the mean flux decreased by 7.8 ± 1.3% for the C II 1334.5323Å and 1335.6854Å lines and by 8.2 ± 1.4% for the Si III 1206.500Å line during transit compared to non-transit times in the velocity interval -50 to +50 km s −1 . Comparison of the C II and Si III line depths and transit/nontransit line ratios shows deeper absorption features near -10 and +15 km s −1 and less certain features near -40 and +30-70 km s −1 , but future observations are needed to verify this first detection of velocity structure in the expanding atmosphere of an exoplanet. Our results for the C II lines and the non-detection of Si IV 1394.76Å absorption are in agreement with Vidal-Madjar et al. (2004), but we find absorption during transit in the Si III line contrary to the earlier result. The 8 ± 1% obscuration of the star during transit is far larger than the 1.5% obscuration by the exoplanet's disk. Absorption during transit at velocities between -50 and +50 km s −1 in the C II and Si III lines requires high-velocity ion absorbers. Assuming hydrodynamic model values for the gas temperature and outflow velocity at the limb of the outflow as seen in the C II lines, we find mass-loss rates in the range (8-40)×10 10 g s −1 . These rates assume that the carbon abundance is solar, which is not the case for the giant planets in the solar system. Our mass-loss rate estimate is consistent with theoretical hydrodynamic models that include metals in the outflowing gas.Subject headings: planets and satellites: atmospheres -planets and satellites: individual (HD 209458b) -planets and satellites: physical evolution -stars: individual (HD 209458) -ultraviolet: stars

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905 -1187 Å with high spectral resolution. The instrument consists of four coaligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al:LiF coatings for optimum reflectivity from approximately 1000 to 1187 Å and the other two use SiC coatings for optimized throughput between 905 and 1105 Å. The gratings are holographically ruled to largely correct 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 as well as active galactic nuclei and QSOs for absorption line studies of both Milky Way and extra-galactic 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.

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