Far Ultraviolet Spectroscopic Explorer Research Articles

FUSE observations of molecular hydrogen on the line of sight towards HD 141569A

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of HD 141569A, a transitional object known to possess a circumstellar disk. We observe two components of gas at widely different temperatures along the line of sight. We detect cold H 2 , which is thermalized up to J = 2 at a kinetic temperature of ∼51 K. Such low temperatures are typical of the diffuse interstellar medium. Since the line of sight to HD 141569A does not pass through its disk. it appears that we are observing the cold H 2 in a low extinction envelope associated with the high Galactic latitude dark cloud complex L134N, which is in the same direction and at nearly the same distance as HD 141569A. The column densities of the higher J-levels of H 2 suggest the presence of warm gas along the line of sight. The excitation conditions do not seem to he consistent with what is generally observed in diffuse interstellar clouds. The observed radial velocity of the gas implies that the UV spectral lines we observe are likely interstellar in origin rather than circumstellar, although our absorption line study does not definitely rule out the possibility that the warm gas is close to the star. The discovery of such warm gas along the line of sight may provide evidence for turbulent phenomena in the dark cloud L134N.

FUSEMeasurements of Far‐Ultraviolet Extinction. II. Magellanic Cloud Sight Lines

We present an extinction analysis of nine reddened/comparison star pairs in the Large and Small Magellanic Clouds (LMC and SMC) based on Far-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Magellanic Clouds using spectral data for wavelengths as short as 950 A. We supplement these two with data from four regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinct characters of extinction in the Clouds and Milky Way, our results are generally analogous to those found for Galactic curves—namely, that the FUSE portions of each extinction curve are described reasonably well by Fitzpatrick & Massa curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the Cardelli, Clayton, & Mathis (CCM) formalism continue into FUV wavelengths. A maximum entropy method analysis of all of these curves suggests that LMCAvg and SMC wing sight lines, whose extinction parameters more closely resemble those for Galactic paths, require more silicon and/or carbon in dust than current abundance measurements would indicate are available. The requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs, in part because large grains contribute less to the extinction in these directions. An intermediate product of this extinction analysis is the measurement of new H2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H2 column densities, E(B - V)/N(H ) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B - V)/N(H2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H2) values are lower than those common in the Milky Way for paths with similar degrees of reddening.

Abundance of Elements beyond the Iron Group in Cool DO White Dwarfs

We report the presence of elements beyond the iron group in the atmospheres of the cool DO white dwarfs HD 149499 B and HZ 21. Photospheric lines of germanium (Z = 32), arsenic (33), selenium (34), tin (50), tellurium (52), iodine (53), and perhaps bromine (35) are observed in ultraviolet spectra of HD 149499 B obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Goddard High Resolution Spectrograph (GHRS), and the International Ultraviolet Explorer. Germanium, arsenic, and tellurium are also observed in FUSE and GHRS spectra of HZ 21. Light elements such as carbon, silicon, phosphorus, and sulfur are present in the atmospheres of both stars. Nitrogen is detected in HZ 21 but not in HD 149499 B. We calculated a grid of synthetic spectra covering the range of effective temperatures, surface gravities, and element abundances appropriate for a detailed line profile analysis. The measured mass fractions of elements heavier than iron reveal overabundances ranging from about a factor of 3 to a factor of 1000 relative to the Sun. The heavy-element enrichment in both stars could be the result of slow neutron capture nucleosynthesis, which would have occurred during the asymptotic giant branch phase of their evolution.

Fluorescent Molecular Hydrogen Emission in IC 63:FUSE, Hopkins Ultraviolet Telescope, and Rocket Observations

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5 IV star γ Cas, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations toward three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a FUSE spectrum of γ Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointings. The intensity of this emission, as well as the contributions from other species, are seen to vary with position. The absolute flux calibration of the sounding rocket data allows us to reliably predict the radiation field incident on IC 63. We use these data to test models of the fluorescent process. Our modeling resolves the perceived discrepancy between the existing ultraviolet observations and achieves a satisfactory agreement with the H2 rotational structure observed with FUSE.

Modelling the chromosphere and transition region of ε Eri (K2 V)

Measurements of ultraviolet line fluxes from Space Telescope Imaging Spectrograph and Far-Ultraviolet Spectroscopic Explorer spectra of the K2-dwarf Epsilon Eri are reported. These are used to develop new emission measure distributions and semi-empirical atmospheric models for the chromosphere and lower transition region of the star. These models are the most detailed constructed to date for a main-sequence star other than the Sun. New ionisation balance calculations, which account for the effect of finite density on dielectronic recombination rates, are presented for carbon, nitrogen, oxygen and silicon. The results of these calculations are significantly different from the standard Arnaud & Rothenflug ion balance, particularly for alkali-like ions. The new atmospheric models are used to place constraints on possible First Ionisation Potential (FIP) related abundance variations in the lower atmosphere and to discuss limitations of single-component models for the interpretation of certain optically thick line fluxes.

Spin-resolved Far-Ultraviolet Observations of the Magnetic White Dwarf in YY Draconis

We observed the intermediate polar YY Draconis with the Far Ultraviolet Spectroscopic Explorer (FUSE), yielding an average far-UV spectrum, as well as spectra and spectrophotometric light curves resolved on the spin phase of the white dwarf in this system. We utilized the BINSYN software suite to calculate model spectra based on a simple system model in which the white dwarf has two identical, two-dimensional, equatorial photospheric hot spots separated by 180° in longitude, which correspond to the poles of a simple centered dipole magnetic field. By optimizing the BINSYN model spectra to match our FUSE data plus archival International Ultraviolet Explorer spectra, we determine a white dwarf effective temperature of 21,500 K with the magnetic pole spots having effective temperatures of 220,000 K and angular radii of 35. Normalization of the model spectra implies a distance to YY Dra of 159 pc. We also construct model light curves resolved on the spin period of the white dwarf via synthetic spectrophotometry of the BINSYN synthetic spectra. These are compared with the light curves constructed from our FUSE data, as well as the Hubble Space Telescope ultraviolet light curves of Haswell et al. In general, we find good agreement between the observed and model spectra and light curves, with the exception of a discrepancy at the shortest wavelengths of the spin-phase minimum light spectrum. This implies the possible presence of vertically extended structure(s) on the white dwarf that are visible above the stellar limb even when the photospheric spots are not and contribute far-UV flux to the observed spectrum that is not reproduced in the BINSYN model. We suggest either the accretion columns above the magnetic poles of the white dwarf or a localized buildup of accreted material on the white dwarf surface at the accretion impact sites as possible sources for the missing far-UV flux. Another possibility invokes a model with a more complex magnetic field geometry, in which the spots are still visible during the spin-phase minimum light configuration, but it is neither conclusive nor well constrained by the available data.

The boundary layer of VW Hydri in quiescence

In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the 'accretion belt'. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer (FUSE) observations of VW Hyi in quiescence, which confirm the presence of a second component (the 'accretion belt') in the far-UV spectrum. The temperature ( 50000 K) and rotational velocity ( 3000 km s -1 ) of this second component are entirely consistent with the optically thick region (r 1) located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to L BL = (0.53 ± 0.25) L disc , while the theory (Kluzniak) predicts, for the rotation rate of VW Hyi's white dwarf, L BL (0.76 ± 0.03)L disc . About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Cordova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of v 10 13 -10 14 cm 2 s -1 , depending on the white dwarf mass and the size of the boundary layer.

Far Ultraviolet Spectroscopic ExplorerObservations of RX J0513.9-6951

Far Ultraviolet Spectroscopic Explorer (FUSE) observations were obtained in 2003 July during 1.2 cycles of the 0.76 day binary orbit of RX J0513.9-6951. Radial velocity measurements of the broad O VI emission profile show a semiamplitude of K ∼ 26 km s-1, which is much smaller than the value of 117 km s-1 measured from 2001 FUSE data. Narrow O VI emissions show no measurable velocity variation. The mean velocity of the broad O VI emission is redshifted by ∼500 km s-1 with respect to both the systemic and narrow emission-line velocities. Spectral difference plots show phase-related changes in the broad emission profile. Other phase-related changes, such as line and continuum variations, are also smaller than those in the 2001 spectra. We describe a moving broad absorption feature near 1020 Å as a possible O VI outflow associated with a precessing jet. We discuss the implications for the stellar masses if the 2003 broad O VI velocities outline the compact star's orbital motion.

TheFUSESpectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the young, compact planetary nebula (PN) SwSt 1 along the line of sight to its central star HD 167362. We detect circumstellar absorption lines from several species against the continuum of the central star. The physical parameters of the nebula derived from the FUSE data differ significantly from those found from emission lines. We derive an electron density ne = 8800 cm-3 from the column density ratio of the excited S III fine-structure levels, which is at least a factor of 3 lower than all prior estimates. The gaseous iron abundance derived from the UV lines is quite high ([Fe/S] = -0.35 ± 0.12), which implies that iron is not significantly depleted into dust. In contrast, optical and near-infrared emission lines indicate that Fe is more strongly depleted: [Fe/H] = -1.64 ± 0.24 and [Fe/S] = -1.15 ± 0.33. We do not detect nebular H2 absorption, to a limit N(H2) < 7 × 1014 cm-2, at least 4 orders of magnitude lower than the column density estimated from infrared H2 emission lines. Taken together, the lack of H2 absorption, low ne, and high gaseous Fe abundance derived from the FUSE spectrum provide strong evidence that dense structures (which can shield molecules and dust from the destructive effects of energetic stellar photons) are not present along the line of sight to the central star. On the other hand, there is substantial evidence for dust, molecular material, and dense gas elsewhere in SwSt 1. Therefore, we conclude that the nebula must have an inhomogeneous structure. We detect nebular absorption at 1040.94 and 1041.69 A from the two excited fine-structure levels of neutral oxygen. These levels give rise to far-infrared emission lines at 63 and 145 μm, which are often used to infer gas properties, particularly temperature, under the assumption that they are collisionally excited. We find that the O I fine-structure levels in SwSt 1 have an inverted population ratio. This requires a nonthermal excitation mechanism, which we identify as fluorescent excitation by the stellar continuum. To the extent that fluorescence affects the level populations, the far-infrared [O I] line strengths cannot be directly used as diagnostics of density and temperature.

An Assessment of the Fe xviii and Fe xix Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, extreme-UV, and far-UV line emission, formed at the peak of Capella's (α Aurigae's) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than a factor of 2 are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common use. In particular, the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton-impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 107 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

A Near‐Solar Metallicity, Nitrogen‐deficient Lyman Limit Absorber Associated with Two S0 Galaxies

The UV spectrum of the bright quasar PHL 1811 at zem = 0.192 reveals a foreground gas system at z = 0.080923 with log N(H ) = 17.98 ± 0.05. We have determined the abundances of various atomic species in this system from a spectrum covering the wavelength range 1160-1730 Å recorded at 7 km s-1 resolution by the E140M grating of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), supplemented by coverage at shorter wavelengths by the Far Ultraviolet Spectroscopic Explorer (FUSE). The abundances of C II, Si II, S II, and Fe II compared to that of O I indicate that a considerable fraction of the gas is in locations where the hydrogen is ionized. An oxygen abundance [O/H] = -0.19 ± 0.08 in the H I-bearing gas indicates that the chemical enrichment of the gas is unusually high for an extragalactic QSO absorption system. However, this same material has an unusually low abundance of nitrogen, [N/O] < -0.59, indicating that there may not have been enough time during this enrichment for secondary nitrogen to arise from low- and intermediate-mass stars. From the convergence of high Lyman series lines we can determine the velocity width of H I, and after correcting for turbulent broadening shown by the O I absorption feature, we derive a temperature T = 7070 K. We determine a lower bound for the electron density n(e) > 10-3 cm-3 by modeling the ionization by the intergalactic radiation field and an upper bound n(e) < 0.07 cm-3 from the absence of much C II in an excited fine-structure level. The thermal pressure in the range 4 cm-3 K < p/k < 140 cm-3 K could be confined by a warm-hot intergalactic medium (WHIM) structure with ρ/ ~ 20 that might accompany a wall of galaxies at the same redshift, seen in data from the Sloan Digital Sky Survey. An r-band image of the field surrounding PHL 1811 recorded by the ACS instrument on HST shows that two galaxies at the same redshift as the gas are S0 galaxies, separated by only 34 and 87 h kpc from the line of sight. One or both of these galaxies may be the source of the material in the Lyman limit system, which may have been expelled from them in a fast wind, by tidal stripping, or by ram pressure stripping. Subtraction of the ACS point-spread function from the image of the QSO reveals the presence of a face-on spiral galaxy under the glare of the quasar; while it is possible that this galaxy may be responsible for the Lyman limit absorption, the exact alignment of the QSO with the center of the galaxy suggests that the spiral is the quasar host.

Highly Ionized High‐Velocity Clouds: Hot Intergalactic Medium or Galactic Halo?

We use spectroscopic data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) to study the wide range of ionization states of the highly ionized high-velocity (HVCs). Studied extensively in O VI absorption, these clouds are usually assumed to be infalling gas in the Galactic halo at distances less than 50 kpc. An alternative model attributes the O VI (and O VII X-ray absorption) to cosmological structures of low-density, shock-heated intergalactic gas, distributed over 1-3 Mpc surrounding the Milky Way. The latter interpretation is unlikely, owing to the enormous required mass of gas (4 × 1012 M☉). Our detection, in 9 of 12 sight lines, of low-ionization stages (C II/III/IV; Si II/III/IV) at similar high velocities as O VI requires gas densities far above that (nH ≈ 5 × 10-6 cm-3) associated with the warm-hot intergalactic medium (WHIM). These HVCs are probably cooling, multiphase gas in the Galactic halo, bow shocks, and interfaces between clouds falling through a hot, rotating gaseous halo. The velocity segregation of these HVCs in Galactic coordinates is consistent with a pattern in which infalling clouds reflect the sense of Galactic rotation, with peculiar velocities superposed.

Highly Ionized Gas in the Galactic Halo and the High‐Velocity Clouds toward PG 1116+215

We have obtained high-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) echelle observations of the quasar PG 1116 + 215 (zem = 0.1765, l = 22336, b = +6821). The semicontinuous coverage of the ultraviolet spectrum over the wavelength range 916-2800 Å provides detections of Galactic and high-velocity cloud (HVC) absorption over a wide range of ionization species: H I, C II-IV, N I-II, O I, O VI, Mg II, Si II-IV, P II, S II, and Fe II over the velocity range -100 km s-1 < vLSR < +200 km s-1. The high dispersion of these spectra (6.5-20 km s-1) reveals that low-ionization species consist of five discrete components: three at low and intermediate velocities (vLSR ≈ -44, -7, +56 km s-1) and two at high velocities (vLSR ≈ +100, +184 km s-1). Over the same velocity range, the higher ionization species (C III-IV, O VI, Si IV)—those with ionization potentials larger than 40 eV—show continuous absorption with column density peaks at vLSR ≈ 10 km s-1, the expected velocity of halo gas corotating with the Galactic disk, and vLSR ≈ +184 km s-1, the velocity of the higher velocity HVC. The velocity coincidence of both low- and high-ionization species in the vLSR ≈ +184 km s-1 HVC gas suggests that they arise in a common structure, though not necessarily in the same gaseous phase. The absorption structure in the high-ionization gas, which extends to very low velocities, suggests a scenario in which a moderately dense cloud of gas is streaming away from the Galaxy through a hot external medium (either the Galactic halo or corona) that is stripping gas from this cloud. The cloud core produces the observed neutral atoms and low-ionization species. The stripped material is the likely source of the high-ionization species. Among the host of collisionally ionized nonequilibrium models, we find that shock ionization and conductive interfaces can account for the column density ratios of high-ionization species. The nominal metallicity of the neutral gas using the O I and H I column densities is [O/H] ∼ -0.66, with a substantial uncertainty caused by the saturation of the H I Lyman series in the FUSE band. The ionization of the cloud core is likely dominated by photons, and assuming the source of ionizing photons is the extragalactic UV background, we estimate the cloud has a density of 10-2.7 cm-3 with a thermal pressure p/k ≈ 24 cm-3 K. If photons escaping the Galactic disk are also included (i.e., if the cloud lies closer than the outer halo), the density and thermal pressure could be higher by as much as 2 dex. In either case, the relative abundances of O, Si, and Fe in the cloud core are readily explained without departures from the solar pattern. We compare the column density ratios of the HVCs toward the PG 1116+215 to other isolated HVCs as well as Complex C. Magellanic Stream gas (either a diffuse extension of the leading arm or gas stripped from a prior passage) is a possible origin for this gas and is consistent with the location of the high-velocity gas on the sky, as well as its high positive velocity, the ionization, and metallicity.

A Far‐Ultraviolet Spectroscopic Survey of Luminous Cool Stars

The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra of eight giant and supergiant stars reveal that high-temperature (3 × 105 K) atmospheres are common in luminous cool stars and extend across the color-magnitude diagram from α Car (F0 II) to the cool giant α Tau (K5 III). Emission present in these spectra includes chromospheric H Lyβ, Fe II, C I, and transition region lines of C III, O VI, Si III, and Si IV. Emission lines of Fe XVIII and Fe XIX signaling temperatures of ~107 K and coronal material are found in the most active stars, β Cet and 31 Com. A short-term flux variation, perhaps a flare, was detected in β Cet during our observation. Stellar surface fluxes of the emission of C III and O VI are correlated and decrease rapidly toward the cooler stars, reminiscent of the decay of magnetically heated atmospheres. Profiles of the C III λ977 lines suggest that mass outflow is underway at T ~ 80,000 K and the winds are warm. Indications of outflow at higher temperatures (3 × 105 K) are revealed by O VI asymmetries and the line widths themselves. High-temperature species are absent in the M supergiant α Ori. Narrow fluorescent lines of Fe II appear in the spectra of many giants and supergiants, apparently pumped by H Lyα, and formed in extended atmospheres. Instrumental characteristics that affect cool star spectra are discussed.

The D/H Ratio toward PG 0038+199

We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG 0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE), with ground-based support from Keck HIRES. We employ curve-of-growth, apparent optical depth, and profile-fitting techniques to measure the column densities and limits of many other species (H2, Na I, C I, C II, C III, N I, N II, O I, Si II, P II, S III, Ar I, and Fe II), which allows us to determine related ratios such as D/O, D/N, and the H2 fraction. Our efforts are concentrated on measuring gas-phase D/H, which is key to understanding Galactic chemical evolution, and comparing it to predictions from big bang nucleosynthesis. We find column densities log N(H ) = 20.41 ± 0.08, log N(D ) = 15.75 ± 0.08, and log N(H2) = 19.33 ± 0.04, yielding a molecular hydrogen fraction of 0.14 ± 0.02 (2 σ errors), with an excitation temperature of 143 ± 5 K. The high H I column density implies that PG 0038+199 lies outside of the Local Bubble; we estimate its distance to be 297 pc (1 σ). [D + HD]/[H + 2H2] toward PG 0038+199 is 1.91 × 10-5 (2 σ). There is no evidence of component structure on the scale of Δv > 8 km s-1, based on Na I, but there is marginal evidence for structure on smaller scales. The D/H value is high compared to the majority of recent D/H measurements but consistent with the values for two other measurements at similar distances. D/O is in agreement with other distant measurements. The scatter in D/H values beyond ~100 pc remains a challenge for Galactic chemical evolution.

O/H in the Local Bubble

We present new measurements of the oxygen gas-phase abundance along four sightlines probing gas inside the Local Bubble, using data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telecope (HST). Combining these with seven previously published N(O I) and N(H I) measurements we determine a mean O/H ratio for the Local Bubble, (O/H) = (3.45 +/- 0.19)x10^(-4) (1 sigma in the mean). Our result is virtually identical to O/H = (3.43 +/- 0.15)x10^(-4) derived from data for sightlines probing gas well outside the Local Bubble. In contrast to the D/O and D/H ratios, which seem to have different values beyond the Local Bubble, our results show that the O/H ratio is constant and single-valued both inside and outside the Local Bubble, for low density sightlines, with column densities up to log N (H) ~ 21. In addition, the similarity of the two values above suggests that the net effect of the uncertain O I f-values in the FUSE bandpass is not significant. Taking into account the latest determinations of the Solar abundance of oxygen, our result implies that ~25% of the oxygen in the Local Bubble is depleted onto dust grains. The similarity of the value for O/H along low average density sightlines in the Local Bubble with that of denser sightlines beyond may permit a limit on the diluting effects of infalling low metallicity gas.

The Hot Intergalactic Medium–Galaxy Connection: Two Strong OviAbsorbers in the Sight Line toward PG 1211+143

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the QSO PG 1211+143 (zem = 0.081) and a galaxy survey of the surrounding field. This sight line shows two strong intergalactic absorption systems at cz 15,300 and 19,300 km s-1. This sight line addresses the nature and origin of the O VI absorbers and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other O VI-bearing absorbers in diverse environments. At 15,300 km s-1, we find four distinct H I components and associated C II, C III, C IV, Si II, Si III, Si IV, N V, and O VI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 h kpc from the sight line. The observed ions of C, Si, and N are likely to be photoionized, but the O VI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly ionized Galactic high-velocity clouds (HVCs); it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s-1, we find five H I components and associated C III, Si III, and collisionally ionized O VI lying 146 h kpc from an isolated galaxy. The properties of the O VI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas (?2%-6% solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

FUSEObservations of Interstellar and Intergalactic Absorption toward the X‐Ray–bright BL Lacertae Object Markarian 421

High-quality Far Ultraviolet Spectroscopic Explorer (FUSE) observations at 20 km s-1 resolution of interstellar and intergalactic absorption from 910 to 1187 A are presented for the X-ray-bright BL Lac object Mrk 421. These observations are supplemented with FUSE data for the distant halo stars BD +38°2182 and HD 93521 near the Mrk 421 line of sight, in order to obtain information about the distance to absorbing structures in the Milky Way toward Mrk 421. The FUSE ISM observations provide measures of absorption by O VI and many other species commonly found in warm neutral and warm ionized gas, including H I, C II, C III, O I, N I, N II, Fe II, and Fe III. In this study we consider the O VI absorption between -140 and 165 km s-1 and its relationship to the lower ionization absorption and strong absorption produced by O VII and O VIII at X-ray wavelengths. The O VI absorption extending from -140 to 60 km s-1 is associated with strong low-ionization gas absorption and originates in the Galactic thick disk/halo. This O VI appears to be produced by a combination of processes, including conductive interfaces between warm and hot gas and possibly cooling Galactic fountain gas and hot halo gas bubbles. The O VI absorption extending from 60 to 165 km s-1 has unusual ionization properties in that there is very little associated low-ionization absorption, with the exception of C III. This absorption is not observed toward the two halo stars, implying that it occurs in gas more distant than 3.5 kpc from the Galactic disk. Over the 60-165 km s-1 velocity range, O VI and C III absorption have the same kinematic behavior. The ratio N(O )/N(C ) = 10 ± 3 over the 60-120 km s-1 velocity range. Given the association of O VI with C III, it is unlikely that the high-velocity O VI coexists with the hotter gas responsible for the O VII and O VIII absorption. The O VI positive velocity absorption wing might be tracing cooler gas entrained in a hot Galactic fountain outflow. The O VII and O VIII absorption observed by Chandra and XMM-Newton may trace the hot gas in a highly extended (~100 kpc) Galactic corona or hot gas in the Local Group. The low resolution of the current X-ray observations (~750-900 km s-1) and the kinematical complexity of the O VI absorption along typical lines of sight through the Milky Way halo make it difficult to clearly associate the O VI absorption with that produced by O VII and O VIII. A search for metal lines associated with the Lyα absorber at z = 0.01, which is situated in a galactic void, was unsuccessful.

Deuterated molecular hydrogen in the Galactic ISM

We present column density measurements of the HD molecule in the interstellar gas toward 17 Galactic stars. The values for the seven most heavily reddened sightlines, with , are derived from observations with the Far Ultraviolet Spectroscopic Explorer (FUSE). The other ten values are from a reanalysis of spectra obtained with Copernicus. In all cases, high-resolution ground-based observations of and/or the CH molecule were used to constrain the gas velocity structure and to correct for saturation effects. Comparisons of the column densities HD, CH, CN, and in these 17 sightlines indicate that HD is most tightly correlated with CH. Stringent lower limits to the interstellar D/H ratio, derived from the HD/2H2 ratio, range from to . Our results also suggest that the HD/H2 ratio increases with the molecular fraction f(H2) and that the interstellar D/H ratio might be obtained from HD by probing clouds with f(H. Finally, we note an apparent relationship between the molecular fractions of hydrogen and deuterium.

Far Ultraviolet Spectroscopic ExplorerObservations of the Symbiotic Star AG Draconis

Spectra of the bright symbiotic star AG Draconis (BD +67 � 922) in the wavelength range 905–1187 8 obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) are presented. The spectra show a number of narrow, nebular emission lines, together with a uniform continuum from the hot component of the system, and numerous interstellar absorption lines. We infer the existence of Ne viiiin the AG Dra nebula through the identification of the Ne vii k973.3 recombination line. The emission-line spectrum is dominated by intense lines of O vi but also shows weaker lines from highly ionized ions including Ne v ,N evi ,S iv ,a nd Svi. Members of the He ii Balmer series can be identified up to n ¼ 20. Lines of Fe ii and Fe iii fluoresced by O vi k1032 are identified at wavelengths 1141.172 and 1142.429 8, respectively. The emission lines are shown to be produced in a plasma with an electron temperature of 20,000–30,000 K, photoionized by the white dwarf. The Ne vi k997/k999 ratio shows that this ion and all others except perhaps Ne vii are formed at least 300 white dwarf radii from the white dwarf. Revised wavelengths for the Ne v 2s 2 2p 23 P0,1–2s2p 35 S2 and Ne vi 2s 2 2p 2 P–2s2p 24 P transitions are published. Subject heading