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The SAURON project - VIII. OASIS/CFHT integral-field spectroscopy of elliptical and lenticular galaxy centres
We present high spatial resolution integral-field spectroscopy of 28elliptical (E) and lenticular (S0) galaxies from the SAURONrepresentative survey obtained with the OASIS spectrograph during itsoperation at the Canada-France-Hawaii Telescope. These seeing-limitedobservations explore the central 8 × 10 arcsec2(typically 1 kpc diameter) regions of these galaxies using a spatialsampling four times higher than SAURON (0.27-arcsec versus 0.94-arcsecspatial elements), resulting in almost a factor of 2 improvement in themedian point spread function. These data allow accurate study of thecentral regions to complement the large-scale view provided by SAURON.Here we present the stellar and gas kinematics, stellar absorption-linestrengths and nebular emission-line strengths for this sample. We alsocharacterize the stellar velocity maps using the `kinemetry' technique,and derive maps of the luminosity-weighted stellar age, metallicity andabundance ratio via stellar population models. We give a brief review ofthe structures found in our maps, linking also to larger-scalestructures measured with SAURON. We present two previously unreportedkinematically decoupled components (KDCs) in the centres of NGC3032 andNGC4382. We compare the intrinsic size and luminosity-weighted stellarage of all the visible KDCs in the full SAURON sample, and find twotypes of components: kiloparsec-scale KDCs, which are older than 8 Gyr,and are found in galaxies with little net rotation; and compact KDCs,which have intrinsic diameters of less than a few hundred parsec, show arange of stellar ages from 0.5 to 15 Gyr (with 5/6 younger than 5 Gyr),are found exclusively in fast-rotating galaxies, and are close tocounter-rotating around the same axis as their host. Of the sevengalaxies in the SAURON sample with integrated luminosity-weighted agesless than 5 Gyr, five show such compact KDCs, suggesting a link betweencounter-rotation and recent star formation. We show that this may be dueto a combination of small sample size at young ages, and anobservational bias, since young KDCs are easier to detect than theirolder and/or corotating counterparts.Based on observations obtained at the Canada-France-Hawaii Telescopewhich is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: mcdermid@strw.leidenuniv.nl
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| The OB binary HD152219: a detached, double-lined, eclipsing system
We present the results of an optical spectroscopic campaign on themassive binary HD152219 located near the core of the NGC6231 cluster.Though the primary to secondary optical brightness ratio is probablyabout 10, we clearly detect the secondary spectral signature and wederive the first reliable SB2 orbital solution for the system. Theorbital period is close to 4.2403d and the orbit is slightly eccentric(e = 0.08 +/- 0.01). The system is most probably formed by an O9.5 giantand a B1-2 V-III star. We derive minimal masses of 18.6 +/- 0.3 and 7.3+/- 0.1Msolar for the primary and secondary, respectively,and we constrain the stellar radius at values about 11 and5Rsolar. INTEGRAL-Optical Monitoring Camera (OMC) data revealthat HD152219 is the third O-type eclipsing binary known in NGC6231. Inthe Hertzsprung-Russell (HR) diagram, the primary component lies on theblue edge of the β Cep-type instability strip and its spectrallines display clear profile variations that are reminiscent of thoseexpected from non-radial pulsations. Finally, we report the analysis ofXMM-Newton observations of the system. The X-ray spectrum is relativelysoft and is well reproduced by a two-temperature mekal model withkT1 = 0.26keV and kT2 = 0.67keV. The X-ray flux ismost probably variable on a time-scale of days. The average X-rayluminosity during our campaign is log(LX) ~ 31.8(ergs-1), but shows fluctuations of about 10 per cent aroundthis value.Based on observations collected at the European Southern Observatory (LaSilla, Chile) and with the XMM-Newton satellite, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA); also based on data from the Optical MonitoringCamera (OMC) Archive at LAEFF, processed by ISDC.E-mail: hsana@eso.org ‡FNRS Research Associate (Belgium).
| High-Resolution X-Ray Spectroscopy of the Interstellar Medium. II. Neon and Iron Absorption Edges
We present high-resolution spectroscopy of the neon K-shell and ironL-shell interstellar absorption edges in nine X-ray binaries using theHigh Energy Transmission Grating Spectrometer (HETGS) on board theChandra X-Ray Observatory. We found that the iron absorption is well fitby an experimental determination of the cross section for metallic iron,although with a slight wavelength shift of ~20 mÅ. The neon edgeregion is best fit by a model that includes the neutral neon edge andthree Gaussian absorption lines. We identify these lines as due to the1s-2p transitions from Ne II, Ne III, and Ne IX. As we found in ouroxygen edge study, the theoretical predictions for neutral andlow-ionization lines all require shifts of ~20 mÅ to match ourdata. Combined with our earlier oxygen edge study, we find that abest-fit O/Ne ratio of 5.4+/-1.6, consistent with standard interstellarabundances. Our best-fit Fe/Ne ratio of 0.20+/-0.03 is significantlylower than the interstellar value. We attribute this difference to irondepletion into dust grains in the interstellar medium. We make the firstmeasurement of the neon ionization fraction in the ISM. We find Ne II/NeI~0.3 and Ne III/Ne I~0.07. These values are larger than is expectedgiven the measured ionization of interstellar helium. For Ne IX, ourresults confirm the detection of the hot ionized interstellar medium ofthe Galaxy.
| Simultaneous photometry and echelle spectroscopy of the dwarf nova BZ Ursae Majoris during the 2005 January outburst
We report simultaneous photometric and echelle-spectroscopicobservations of the dwarf nova BZ UMa during which we were lucky to catchthe system at the onset of an outburst, the development of which wetraced in detail from quiescence to early decline. The outburst had aprecursor, and was of a short duration (~5d) with a highly asymmetricallight curve. On the rise we observed a `jump' during which thebrightness almost doubled over the course of half an hour. Power spectraanalysis revealed well-defined oscillations with period of ~42min. UsingDoppler tomography we found that the unusual emission distributiondetected in quiescence held during the outburst. After the maximum a newemission source arose, from the inner hemisphere of the secondary star,which became the brightest at that time. We analyse this outburst interms of `inside-out' and `outside-in' types, in order to determinewhich of these types occurred in BZ UMa.
| The SPEAR Instrument and On-Orbit Performance
The SPEAR (or ``FIMS'') instrumentation has been used to conduct thefirst large-scale spectral mapping of diffuse cosmic far-ultraviolet(FUV; 900-1750 Å) emission, including important diagnostics ofinterstellar hot (104-106 K) and photoionizedplasmas, H2, and dust-scattered starlight. Theinstrumentation's performance has allowed for the unprecedenteddetection of astrophysical diffuse FUV emission lines. A spectralresolution of λ/Δλ~550 and an imaging resolution of5' is achieved on-orbit in the Short (900-1150 Å) and Long(1350-1750 Å) bandpass channels within their respective4.0d×4.6 arcmin and 7.4d×4.3 arcmin fields of view. Wedescribe the SPEAR imaging spectrographs, their performance, and thenature and handling of their data.
| Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit
Massive X-ray binaries are usually classified by the properties of thedonor star in classical, supergiant and Be X-ray binaries, the maindifference being the mass transfer mechanism between the two components.The massive X-ray binary 4U 2206+54 does not fit inany of these groups, and deserves a detailed study to understand how thetransfer of matter and the accretion on to the compact object takeplace. To this end we study an IUE spectrum of the donor and obtain awind terminal velocity (v_&infy;) of ~350 km s-1, which isabnormally slow for its spectral type. We also analyse here more than 9years of available RXTE/ASM data. We study the long-term X-rayvariability of the source and find it to be similar to that observed inthe wind-fed supergiant system Vela X-1, reinforcingthe idea that 4U 2206+54 is also a wind-fed system.We find a quasi-period decreasing from ~270 to ~130 d, noticed inprevious works but never studied in detail. We discuss possiblescenarios for its origin and conclude that long-term quasi-periodicvariations in the mass-loss rate of the primary are probably drivingsuch variability in the measured X-ray flux. We obtain an improvedorbital period of P_orb=9.5591±0.0007 d with maximum X-ray fluxat MJD 51856.6±0.1. Our study of the orbital X-ray variability inthe context of wind accretion suggests a moderate eccentricity around0.15 for this binary system. Moreover, the low value of v_&infy; solvesthe long-standing problem of the relatively high X-ray luminosity forthe unevolved nature of the donor, BD +53°2790,which is probably an O9.5 V star. We note that changes in v_&infy;and/or the mass-loss rate of the primary alone cannot explain thedifferent patterns displayed by the orbital X-ray variability. Wefinally emphasize that 4U 2206+54, together withLS 5039, could be part of a new population ofwind-fed HMXBs with main sequence donors, the natural progenitors ofsupergiant X-ray binaries.
| The Remarkable Be Star HD 110432 (BZ Crucis)
HD 110432 (B1e) has gained considerable recent attention because it is ahard, variable X-ray source with local absorption and also because itsoptical spectrum is affected by an extensive Be disk. From time-serialechelle data obtained over 2 weeks during 2005 January and February, wehave discovered several remarkable characteristics in the star's opticalspectrum. The line profiles show rapid variations on some nights, whichcan most likely be attributed to irregularly occurring and short-livedmigrating subfeatures. Such features have been found in spectra ofγ Cas and AB Dor, two stars for which it is believed magneticfields force circumstellar clouds to corotate over the star's surface.The star's optical spectrum also exhibits a number of mainly Fe II andHe I emission features with double-lobed profiles typical of anoptically thin circumstellar disk viewed nearly edge-on. Using spectralsynthesis techniques for the January data, we find that its temperatureand column density are close to 9800 K and roughly3×1022 cm-2, respectively. Its projecteddisk size covers remarkably large 100 stellar areas, and the emittingvolume resides at a surprisingly large distance of 1 AU from the star.Surprisingly, we also find that the absorption wings of the strongestoptical and UV lines in the spectrum extend to at least +/-1000 kms-1, even though the rotational velocity is 300-400 kms-1. We are unable to find a satisfactory explanation forthese extreme line broadenings. Otherwise, HD 110432 and γ Casshare similarly peculiar X-ray and optical characteristics. Theseinclude a high X-ray temperature, erratic X-ray variability ontimescales of a few hours, optical metallic emission lines, andsubmigrating features in optical line profiles. Because of thesesimilarities, we suggest that HD 110432 is a member of a select newclass of ``γ Cas analogs.''
| Variability of the HeI line profiles in the spectrum of the hot O9.5V star HD 93521
CCD spectra acquired with the PFES echelle spectrograph on the 6 mtelescope of the Special Astrophysical Observatory (Russian Academy ofSciences) were used to study short-term variations in the HeI-lineprofiles in the spectrum of HD 93521. For all the lines, the variabilitypattern relative to the mean profile is the same, and can be describedas a sinusoidal wave passing through the profiles, from the blue to thered wings. The variability amplitudes and time scales are different fordifferent HeI lines. We studied variations of the radial velocities atthe level of 0.5 R 0 of the line residual intensity, for the absorptionbisector and the blue and red halves of the absorption profile. Thevariation time scales and amplitudes for the line halves differ from oneHeI line to another, and show good correlations with the line centraldepths. Going from the weak to the strong lines, the time scale of theradial-velocity variations measured for both halves of the absorptionprofile increases, and the amplitude decreases. The time scale of theradial-velocity variations for weak lines is, on average, twice the timescale for strong HeI lines. A variable absorption feature was detectedin the profiles of strong HeI lines, which moves across the profilesynchronously with the star’s axial rotation. Generally, theobserved line variations are probably due to nonradial photosphericpulsations, together with the influence of the stellar wind on theprofiles of the strong lines.
| A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo
We describe a new method for accurately determining total gas-phaseabundances for the Galactic halo interstellar medium with minimalionization uncertainties. For sight lines toward globular clusterscontaining both ultraviolet-bright stars and radio pulsars, it ispossible to measure column densities of H I and several ionizationstates of selected metals using ultraviolet absorption line measurementsand of H II using radio dispersion measurements. By measuring theionized hydrogen column, we minimize ionization uncertainties thatplague abundance measurements of Galactic halo gas. We apply this methodfor the first time to the sight line toward the globular cluster Messier3 [(l,b)=(42.2d,+78.7d), d=10.2 kpc, z=10.0 kpc] using Far UltravioletSpectroscopic Explorer and Hubble Space Telescope ultravioletspectroscopy of the post-asymptotic giant branch star von Zeipel 1128and radio observations by Ransom et al. of recently discoveredmillisecond pulsars. The fraction of hydrogen associated with ionizedgas along this sight line is 45%+/-5%, with the warm (T~104K) and hot (T>~105 K) ionized phases present in roughly a5:1 ratio. This is the highest measured fraction of ionized hydrogenalong a high-latitude pulsar sight line. We derive total gas-phaseabundances logN(S)/N(H)=-4.87+/-0.03 and logN(Fe)/N(H)=-5.27+/-0.05. Ourderived sulfur abundance is in excellent agreement with recent solarsystem determinations of Asplund, Grevesse, & Sauval. However, it is-0.14 dex below the solar system abundance typically adopted in studiesof the interstellar medium. The iron abundance is ~-0.7 dex below thesolar system abundance, consistent with the significant incorporation ofiron into interstellar grains. Abundance estimates derived by simplycomparing S II and Fe II to H I are +0.17 and +0.11 dex higher,respectively, than the abundance estimates derived from our refinedapproach. Ionization corrections to the gas-phase abundances measured inthe standard way are, therefore, significant compared with themeasurement uncertainties along this sight line. The systematicuncertainties associated with the uncertain contribution to the electroncolumn density from ionized helium could raise these abundances by<~+0.03 dex (+7%). Uncertainties in the amount of very hot gas(T~106 K) along the line of sight could also affect thesedeterminations.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Probing the Dust Responsible for Small Magellanic Cloud Extinction
We explore the abundances and depletion levels of elements important todust composition in four Small Magellanic Cloud (SMC) sight linesobserved with the Hubble Space Telescope. The apparent optical depthmethod is used to interpret the high-resolution ultraviolet absorptiondata toward AzV 18, AzV 456, Sk 108, and Sk 155; the latter two sightlines have been previously explored using component fitting. In contrastto previous studies, we find evidence that silicon is depleted in theSMC's interstellar medium (ISM). This makes it improbable that SMC-likeextinction, i.e., no 2175 Å bump and a steep far-ultraviolet rise,results from a lack of silicate grains in the SMC. Notable differencesbetween our sight lines that have SMC-like and Milky Way-likeextinction, AzV 18 and AzV 456, respectively, are that the former has asubstantially larger gas-to-dust ratio and smaller fractionalH2 abundance. Iron abundances and depletions in the SMC's ISMoften diverge from the pattern shown by Si and Mg. This is evidence thatFe is not tied to the same grains as silicon, and therefore that mostsilicate grains are likely magnesium based. The presence of irondepletion in the SMC then suggests that this element is probablyincorporated into grain types such as metals or oxides.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| A Cross-Calibration between Tycho-2 Photometry and Hubble Space Telescope Spectrophotometry
By analyzing a well-calibrated sample of 256 stars observed with bothHipparcos and HST, I show that Tycho-2 photometry and HSTspectrophotometry are accurate and stable enough to obtain a precisecross-calibration. Based on this analysis, I obtain the followingphotometric zero points with respect to Vega for Tycho-2: 0.020+/-0.001(BT-VT), 0.078+/-0.009 (BT), and0.058+/-0.009 (VT).
| The Massive Runaway Stars HD 14633 and HD 15137
We present results from a radial velocity study of two runaway O-typestars, HD 14633 (ON8.5 V) and HD 15137 [O9.5 III(n)]. We find that HD14633 is a single-lined spectroscopic binary with an orbital period of15.4083 days. The second target, HD 15137, is a radial velocity variableand a possible single-lined spectroscopic binary with a period close to1 month. Both binaries have large eccentricity, small semiamplitude, anda small mass function. We show the trajectories of the stars in the skybased on an integration of motion in the Galactic potential, and wesuggest that both stars were ejected from the vicinity of the opencluster NGC 654 in the Perseus spiral arm. The binary orbital parametersand runaway velocities are consistent with the idea that both thesestars were ejected by supernova explosions in binaries and that theyhost neutron star companions. We find that the time of flight sinceejection is longer than the predicted evolutionary timescales for thestars. This discrepancy may indicate that the stars have a lower massthan normally associated with their spectral classifications, that theywere rejuvenated by mass transfer prior to the supernova, or that theirlives have been extended through rapid rotation.Based in part on observations made at the Observatoire de Haute Provence(CNRS), France.
| FUSE Observations of Interstellar and Intergalactic Absorption toward the X-Ray-bright BL Lacertae Object Markarian 421
High-quality Far Ultraviolet Spectroscopic Explorer (FUSE) observationsat 20 km s-1 resolution of interstellar and intergalacticabsorption from 910 to 1187 Å are presented for the X-ray-brightBL Lac object Mrk 421. These observations are supplemented with FUSEdata for the distant halo stars BD +38°2182 and HD 93521 near theMrk 421 line of sight, in order to obtain information about the distanceto absorbing structures in the Milky Way toward Mrk 421. The FUSE ISMobservations provide measures of absorption by O VI and many otherspecies commonly found in warm neutral and warm ionized gas, including HI, C II, C III, O I, N I, N II, Fe II, and Fe III. In this study weconsider the O VI absorption between -140 and 165 km s-1 andits relationship to the lower ionization absorption and strongabsorption produced by O VII and O VIII at X-ray wavelengths. The O VIabsorption extending from -140 to 60 km s-1 is associatedwith strong low-ionization gas absorption and originates in the Galacticthick disk/halo. This O VI appears to be produced by a combination ofprocesses, including conductive interfaces between warm and hot gas andpossibly cooling Galactic fountain gas and hot halo gas bubbles. The OVI absorption extending from 60 to 165 km s-1 has unusualionization properties in that there is very little associatedlow-ionization absorption, with the exception of C III. This absorptionis not observed toward the two halo stars, implying that it occurs ingas more distant than 3.5 kpc from the Galactic disk. Over the 60-165 kms-1 velocity range, O VI and C III absorption have the samekinematic behavior. The ratio N(OVI)/N(CIII)=10+/-3 over the 60-120 kms-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 gasresponsible for the O VII and O VIII absorption. The O VI positivevelocity absorption wing might be tracing cooler gas entrained in a hotGalactic fountain outflow. The O VII and O VIII absorption observed byChandra and XMM-Newton may trace the hot gas in a highly extended (~100kpc) Galactic corona or hot gas in the Local Group. The low resolutionof the current X-ray observations (~750-900 km s-1) and thekinematical complexity of the O VI absorption along typical lines ofsight through the Milky Way halo make it difficult to clearly associatethe O VI absorption with that produced by O VII and O VIII. A search formetal lines associated with the Lyα absorber at z=0.01, which issituated in a galactic void, was unsuccessful.
| Rocket and Far Ultraviolet Spectroscopic Explorer Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflectionnebula IC 405 obtained by a rocket-borne long-slit spectrograph and theFar Ultraviolet Spectroscopic Explorer (FUSE). Both data sets show arise in the ratio of the nebular surface brightness to stellar flux(S/F*) of approximately 2 orders of magnitude toward the blueend of the far-UV bandpass. Scattering models using simple dustgeometries fail to reproduce the observed S/F* for realisticgrain properties. The high spectral resolution of the FUSE data revealsa rich fluorescent molecular hydrogen spectrum ~1000" north of the starthat is clearly distinguished from the steady blue continuum. TheS/F* remains roughly constant at all nebular pointings,showing that fluorescent molecular hydrogen is not the dominant cause ofthe blue rise. We discuss three possible mechanisms for the ``bluedust'': differential extinction of the dominant star (HD 34078), unusualdust-grain properties, and emission from nebular dust. We conclude thatuncertainties in the nebular geometry and the degree of dust clumpingare most likely responsible for the blue rise. As an interestingconsequence of this result, we consider how IC 405 would appear in aspatially unresolved observation. If IC 405 were observed with a spatialresolution of less than 0.4 pc, for example, an observer would infer afar-UV flux that was 2.5 times the true value, giving the appearance ofa stellar continuum that was less extinguished than radiation from thesurrounding nebula, an effect that is reminiscent of the observedultraviolet properties of starburst galaxies.
| C II Radiative Cooling of the Diffuse Gas in the Milky Way
The heating and cooling of the interstellar medium (ISM) allow the gasin the ISM to coexist at very different temperatures in thermal pressureequilibrium. The rate at which the gas cools or heats is therefore afundamental ingredient for any theory of the ISM. The heating cannot bedirectly determined, but the cooling can be inferred from observationsof .CII*, which is an important coolant in differentenvironments. The amount of cooling can be measured through either theintensity of the 157.7 μm [C II] emission line or the CII*absorption lines at 1037.018 and 1335.708 Å, observable with theFar Ultraviolet Spectroscopic Explorer and the Space Telescope ImagingSpectrograph on board the Hubble Space Telescope, respectively. Wepresent the results of a survey of these far-UV absorption lines in 43objects situated at |b|>~30deg. Measured column densitiesof CII*, S II, P II, and Fe II are combined with H I 21 cmemission measurements to derive the cooling rates (per H atom using H Iand per nucleon using S II) and to analyze the ionization structure,depletion, and metallicity content of the low-, intermediate-, andhigh-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. Based on the depletion and the ionization structure, the LVCs,IVCs, and HVCs consist mostly of warm neutral and ionized clouds. Forthe LVCs, the mean cooling rate in ergs s-1 per H atom is-25.70+0.19-0.36 dex (1 σ dispersion). Witha smaller sample and a bias toward high H I column density, the coolingrate per nucleon is similar. The corresponding total Galactic C IIluminosity in the 157.7 μm emission line isL~2.6×107 Lsolar. CombiningN(CII*) with the intensity of Hα emission, we derivethat ~50% of the CII* radiative cooling comes from the warmionized medium (WIM). The large dispersion in the cooling rates iscertainly due to a combination of differences in the ionizationfraction, in the dust-to-gas fraction, and physical conditions betweensight lines. For the IVC Intermediate-Velocity (IV) Arch at z~1 kpc wefind that on average the cooling is a factor of 2 lower than in the LVCsthat probe gas at lower z. For an HVC (complex C, at z>6 kpc) we findthe much lower rate of -26.99+0.21-0.53 dex,similar to the rates observed in a sample of damped Lyα absorbersystems (DLAs). The fact that in the Milky Way a substantial fraction ofthe C II cooling comes from the WIM implies that this is probably alsotrue in the DLAs. We also derive the electron density, assuming atypical temperature of the warm gas of 6000 K: for the LVCs,=0.08+/-0.04 cm-3, and for the IV Arch,=0.03+/-0.01 cm-3 (1 σdispersion). Finally, we measured the column densities N(S II) and N(PII) in many sight lines and confirm that sulphur appears undepleted inthe ISM. Phosphorus becomes progressively more deficient whenlogN(HI)>19.7 dex, which can mean that either P becomes more depletedinto dust as more neutral gas is present or P is always depleted byabout -0.3 dex, but the higher value of P II at lower H I column densityindicates the need for an ionization correction.
| High-Resolution X-Ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge
We present high-resolution spectroscopy of the oxygen K-shellinterstellar absorption edge in seven X-ray binaries using the HighEnergy Transmission Grating Spectrometer (HETGS) onboard the ChandraX-Ray Observatory. Using the brightest sources as templates, we found abest-fit model of two absorption edges and five Gaussian absorptionlines. All of these features can be explained by the recent predictionsof K-shell absorption from neutral and ionized atomic oxygen. Weidentify the Kα and Kβ absorption lines from neutral oxygen,as well as the S=3/2 absorption edge. The expected S=1/2 edge is notdetected in these data because of overlap with instrumental features. Wealso identify the Kα absorption lines from singly and doublyionized oxygen. The O I Kα absorption line is used as a benchmarkwith which to adjust the absolute wavelength scale for theoreticalpredictions of the absorption cross sections. We find that shifts of30-50 mÅ are required, consistent with differences previouslynoticed from comparisons of the theory with laboratory measurements.Significant oxygen features from dust or molecular components, assuggested in previous studies, are not required by our HETGS spectra.With these spectra, we can begin to measure the large-scale propertiesof the interstellar medium (ISM). We place a limit on the velocitydispersion of the neutral lines of 〈~200 km s-1,consistent with measurements at other wavelengths. We also make thefirst measurement of the oxygen ionization fractions in the ISM. Weconstrain the interstellar ratio of O II/O I to ~0.1 and the ratio of OIII/O I to 〈~0.1. This work demonstrates the utility of X-rayspectroscopy for studies of the ISM. Future work will providemeasurements of the relative abundances and ionization fractions forelements from carbon to iron.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| The metallicity-luminosity relation at medium redshift based on faint CADIS emission line galaxies
The emission line survey within the Calar Alto Deep Imaging Survey(CADIS) detects galaxies with very low continuum brightness by using animaging Fabry-Perot interferometer. With spectroscopic follow-upobservations of MB ⪆ -19 CADIS galaxies using FORS2 atthe VLT and DOLORES at TNG we obtained oxygen abundances of 5 galaxiesat z 0.4 and 10 galaxies at z 0.64. Combining thesemeasurements with published oxygen abundances of galaxies withMB ⪉ -19 we find evidence that a metallicity-luminosityrelation exists at medium redshift, but it is displaced to lowerabundances and higher luminosities compared to themetallicity-luminosity relation in the local universe. Comparing theobserved metallicities and luminosities of galaxies at z ⪉ 3 withPégase2 chemical evolution models we have found a favouredscenario in which the metallicity of galaxies increases by a factor of2 between z 0.7 and today, and their luminosity decreases by0.5-0.9 mag.Based on observations obtained at the ESO VLT, Paranal, Chile; ESOprograms 67.A-0175, 68.B-0088, and 69.A-0266.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Polarimetric evolution of V838 Monocerotis
We present the results of our polarimetric and spectropolarimetricmonitoring of V838 Monocerotis, performed at the Asiago and Crimeanobservatories during and after the multiple outbursts that occurred inJanuary-March 2002. The polarization of the object is mainly due tointerstellar polarization (P 2.48%). Intrinsic polarization up to 0.7% at 5000 Å is present during the second maximum of theobject (February 2002). This intrinsic component increases towardshorter wavelengths but our limited spectral coverage (5000-7500Å) does not allow conclusive inferences about its origin. A strongdepolarization across the Hα profile is observed. Theinterstellar polarization gives a lower limit to the reddening of E(B-V)>0.28, with E(B-V) 0.5 being the most probable value. A normalratio of total to selective absorption (RV=3.22±0.17)was derived from the wavelength of maximum interstellar polarization.This suggests a low (if any) contribution by circumstellar material witha peculiar dust to gas ratio. A polarimetric map of a portion of thelight echo shows a complex polarization distribution reachingPmax=45%.Based on observations collected at Padua Astronomical Observatory atAsiago and Crimean Astrophysical Observatory.
| Probing the inner halo and IVC gas through the Local Interstellar Chimney
We present an absorption study of the interstellar gas at high positivegalactic latitudes in the direction close to the axis of the LocalChimney (LC), which is an extension of the rarefied local cavity thatreaches out from the galactic disk to a z-distance of at least 250 pcinto the lower halo region. Our study includes high-resolution (R 1.7 km s-1) spectral observations of the interstellar NaI andCaII absorption lines seen towards 6 early-type stars with distancesranging from 225 to 500 pc contained within a radius of6° along the sight-line (l = 160°, b= +55°). These visible data are supplemented withfar-ultraviolet absorption measurements of the interstellar sight-linestowards two hot white dwarf stars, RE J1043+490 (d = 230 pc) and REJ1059+512 (d = 315 pc), taken with the NASA Far UltravioletSpectroscopic Explorer (FUSE) satellite.Our observations reveal interstellar gas clouds with velocities in the-20 to -60 km s-1 range that appear to be falling towards thegalactic disk. In particular, we have detected absorption with avelocity of V_helio -55 km s-1 towards two sight-lines(HD 89501 and HD 88545) that can be associated with an intermediatevelocity (IV) cloud called the IV Arch. Our observations place aprobable z-distance to this IVC of 275-320 pc, this being much nearerthan previously thought.The far UV spectra of the two hot white dwarfs show only a few (10)interstellar absorption lines, which is indicative of the very lowdensity phase of the interstellar gas contained within the LC region. Wehave derived relative elemental abundance ratios for C, N, O, Si, Ar andFe for these two sight-lines and find that the abundance patterns arevery similar to those found for other sight-lines in the local cavity.The high ionization line of OVIłambda1032 Å has beendetected towards RE J1043+490 with a column density of 7 ×1012 cm-2, which is consistent with the averagespace density of this ion recently found in the Local Bubble region.Finally, we (tentatively) propose a possible connection between theformation of the Gould Belt, the Local Bubble cavity, the Local Chimneyand the overlying IV Arch clouds.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| Line temperatures and elemental abundances in H II galaxies
We present long-slit spectrophotometric observations in the red andnear-infrared of 12 HII galaxies. The spectral range includes thesulphur lines [SII] at λλ6716,6731 and [SIII] atλ6312 and λλ9069,9532. For all of the observedgalaxies, at least three ion-weighted temperatures from forbiddenauroral to nebular line ratios have been obtained and the relationsbetween the different line temperatures have been discussed. It is foundthat, for some objects, the [OII] temperatures derived from those of[OIII] through the use of photoionization models, without taking intoaccount the effect of density, can lead to a significant underestimateof the O+/H+ ionic abundance and hence of thetotal oxygen abundance.For all the observed objects, we have calculated the ionic abundances ofO+, O2+, S+, S2+ andN+ and they have been used to constrain the ionizationstructure of the emitting regions with the help of photoionizationmodels. From them, the ionization correction factors for N and S, andtheir corresponding total abundances, have been derived.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Neutral Atomic Phases of the Interstellar Medium in the Galaxy
Much of the interstellar medium in disk galaxies is in the form ofneutral atomic hydrogen, H I. This gas can be in thermal equilibrium atrelatively low temperatures, T〈~300 K (the cold neutral medium[CNM]), or at temperatures somewhat less than 104 K (the warmneutral medium [WNM]). These two phases can coexist over a narrow rangeof pressures, Pmin〈=P〈=Pmax. We determinePmin and Pmax in the plane of the Galaxy as afunction of Galactocentric radius R using recent determinations of thegas heating rate and the gas-phase abundances of interstellar gas. Weprovide an analytic approximation for Pmin as a function ofmetallicity, far-ultraviolet radiation field, and the ionization rate ofatomic hydrogen. Our analytic results show that the existence ofPmin, or the possibility of a two-phase equilibrium,generally requires that H+ exceed C+ in abundanceat Pmin. The abundance of H+ is set by EUV/softX-ray photoionization and by recombination with negatively chargedpolycyclic aromatic hydrocarbons. In order to assess whether thermal orpressure equilibrium is a realistic assumption, we define a parameterΥ≡tcool/tshock, where tcoolis the gas cooling time and tshock is the characteristicshock time or ``time between shocks in a turbulent medium.'' ForΥ〈1 gas has time to reach thermal balance betweensupernova-induced shocks. We find that this condition is satisfied inthe Galactic disk, and thus the two-phase description of theinterstellar H I is approximately valid even in the presence ofinterstellar turbulence. Observationally, the mean densityHI〉 is often better determined than the local density, andwe cast our results in terms of HI〉 as well. Over most ofthe disk of the Galaxy, the H I must be in two phases: the weight of theH I in the gravitational potential of the Galaxy is large enough togenerate thermal pressures exceeding Pmin, so that turbulentpressure fluctuations can produce cold gas that is thermally stable; andthe mean density of the H I is too low for the gas to be all CNM. Ourmodels predict the presence of CNM gas to R~=16-18 kpc, somewhat fartherthan previous estimates. An estimate for the typical thermal pressure inthe Galactic plane for 3kpc〈~R〈~18 kpc isPth/k~=1.4×104exp(-R/5.5kpc) Kcm-3. At the solar circle, this gives Pth/k~=3000K cm-3. We show that this pressure is consistent with the CI*/C Itot ratio observed by Jenkins & Tripp and the CNMtemperature found by Heiles & Troland. We also examine the potentialimpact of turbulent heating on our results and provide parameterizedexpressions for the heating rate as a function of Galactic radius.Although the uncertainties are large, our models predict that includingturbulent heating does not significantly change our results and thatthermal pressures remain above Pmin to R~=18 kpc.
| Highly Ionized Gas in the Galactic Halo: A FUSE Survey of O VI Absorption toward 22 Halo Stars
Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactichalo stars are studied to determine the amount of O VI in the Galactichalo between ~0.3 and ~10 kpc from the Galactic midplane. Strong O VIλ1031.93 absorption was detected toward 21 stars, and a reliable3 σ upper limit was obtained toward HD 97991. The weaker member ofthe O VI doublet at 1037.62 Å could be studied toward only sixstars because of stellar and interstellar blending problems. Themeasured logarithmic total column densities vary from 13.65 to 14.57with =14.17+/-0.28 (1 σ). The observed columns arereasonably consistent with a patchy exponential O VI distribution with amidplane density of 1.7×10-8 cm-3 and scaleheight between 2.3 and 4 kpc. We do not see clear signs of stronghigh-velocity components in O VI absorption along the Galactic sightlines, which indicates the general absence of high-velocity O VI within2-5 kpc of the Galactic midplane. This result is in marked contrast tothe findings of Sembach et al., who reported high-velocity O VIabsorption toward ~60% of the complete halo sight lines observed byFUSE. The line centroid velocities of the O VI absorption do not reflectGalactic rotation well. The O VI velocity dispersions range from 33 to78 km s-1, with an average of =45+/-11 kms-1 (1 σ). These values are much higher than the valueof ~18 km s-1 expected from thermal broadening for gas atT~3×105 K, the temperature at which O VI is expected toreach its peak abundance in collisional ionization equilibrium.Turbulence, inflow, and outflow must have an effect on the shape of theO VI profiles. Kinematical comparisons of O VI with Ar I reveal thateight of 21 sight lines are closely aligned in LSR velocity(|ΔVLSR|<=5 km s-1), while nine of 21exhibit significant velocity differences(|ΔVLSR|>=15 km s-1). This dual behaviormay indicate the presence of two different types of O VI-bearingenvironments toward the Galactic sight lines. The correlation betweenthe H I and O VI intermediate-velocity absorption is poor. We couldidentify the known H I intermediate-velocity components in the Ar Iabsorption but not in the O VI absorption in most cases. Comparison of OVI with other highly ionized species suggests that the high ions areproduced primarily by cooling hot gas in the Galactic fountain flow andthat turbulent mixing also has a significant contribution. The role ofturbulent mixing varies from negligible to dominant. It is mostimportant toward sight lines that sample supernova remnants like Loops Iand IV. The average N(C IV)/N(O VI) ratios for the nearby halo (thiswork) and complete halo (Savage et al.) are similar (~0.6), but thedispersion is larger in the sample of nearby halo sight lines. We areable to show that the O VI enhancement toward the Galactic center regionthat was observed in the FUSE survey of complete halo sight lines(Savage et al.) is likely associated with processes occurring near theGalactic center by comparing the observations toward the nearby HD177566 sight line to those toward extragalactic targets.
| Constraints on the Ionization Balance of Hot-Star Winds from FUSE Observations of O Stars in the Large Magellanic Cloud
We present Far Ultraviolet Spectroscopic Explorer (FUSE) spectra for 25O stars in the Large Magellanic Cloud. We analyze wind profiles for theresonance lines from C III, N III, S IV, P V, S VI, and O VI in the FUSErange using a ``Sobolev with exact integration'' method. In addition,the available data from either IUE or the Hubble Space Telescope for theresonance lines of Si IV, C IV, and N V are also modeled. Becauseseveral of the FUSE wind lines are unsaturated, the analysis providesmeaningful optical depths (or equivalently, mass-loss rate timesionization fractions, Mq) as a function of normalized velocity,w=v/v&infy;. Ratios of Mq (which are independent of M)determine the behavior of the relative ionization as a function of w.The results demonstrate that, with the exception of O VI in all starsand S VI in the later stars, the ionization in the winds shifts towardlower ionization stages at higher w (contrary to the expectations of thenebular approximation). This result implies that the dominant productionmechanism for O VI and S VI in the late O stars differs from the otherions. Using the Vink et al. relationship between stellar parameters andmass-loss rate, we convert the measurements into mean ionizationfractions for each ion, 〈qi〉. Because the derived ionfractions never exceed unity, we conclude that the derived values of Mare not too small. However, q(P V), which is expected to be the dominantstage of ionization in some of these winds, is never greater than 0.20.This implies that either the calculated values of M are too large, theassumed abundance of phosphorus is too large, or the winds are stronglyclumped. The implications of each possibility are discussed.Correlations between the mean ion fractions and physical parameters suchas Teff, v&infy;, and the mean wind density,〈ρ〉, are examined. Two clear relationships emerge. First, asexpected, the mean ionization fraction of the lower ions (C III, N III,Si IV, S IV) decreases with increasing Teff. Second, the meanion fraction of S VI in the latest stars and O VI in all stars increaseswith increasing v&infy;. This reaffirms the notion, firstintroduced by Cassinelli & Olson, that O VI is producednonradiatively. Finally, we discuss specific characteristics of threestars, BI 272, BI 208, and Sk -67°166. For BI 272, the ionic speciespresent in its wind suggest it is much hotter than its available(uncertain) spectral type of O7: II-III:. In the case of BI 208, ourinability to fit its observed profiles suggests that its wind is notspherically symmetric. For Sk -67°166, quantitative measurements ofits line strengths confirm the suggestion by Walborn et al. that it is anitrogen-rich O star.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofthe post-asymptotic giant branch star von Zeipel 1128 (l=42.5d, b=+78.7dd=10.2 kpc; z=10.0 kpc vLSR=-140+/-8 km s-1),located in the globular cluster Messier 3. The FUSE observations coverthe wavelength range 905-1187 Å at ~20 km s-1 (FWHM)resolution. These data exhibit many photospheric and interstellarabsorption lines, including absorption from ions associated with thewarm neutral, warm ionized, and highly ionized phases of theinterstellar medium along this sight line. We derive interstellar columndensities of H I, P II, Ar I, Fe II, Fe III, S III, and O VI, with lowerlimits for C II, C III, N I, O I, and Si II. Though the individualvelocity components within the absorption profiles are unresolved byFUSE, a comparison of the velocity distribution of depleted or ionizedspecies with the neutral species suggests that the thick-disk materialalong this sight line is infalling onto the Galactic plane, whilematerial near the plane is seen closer to rest velocities. Ionizedhydrogen represents >~12%, most likely ~45%, of the total hydrogencolumn along this sight line, most of it associated with the warmionized phase. The warm ionized and neutral media toward von Zeipel 1128have very similar gas-phase abundances and kinematics: the neutral andionized gases in this region of the thick disk are closely related.Strong O VI absorption is seen with the same central velocity as thewarm ionized gas, although the O VI velocity dispersion is much higher(σ≡sqrt(2)b=32 km s-1). Virtually all of the O VIis found at velocities where lower ionization gas is seen, suggestingthat the O VI and WNM/WIM probes are tracing different portions of thesame structures (e.g., the O VI may reside in interfaces surrounding theWNM/WIM clouds). We see no evidence for interstellar absorptionassociated with the globular cluster Messier 3 itself or with thecircumstellar environment of von Zeipel 1128. Neither high-velocitycloud absorption (with |vLSR|>~125 km s-1) norhigh velocity dispersion gas (with σ~60 km s-1) is seentoward von Zeipel 1128.
| A Far Ultraviolet Spectroscopic Explorer Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
Far Ultraviolet Spectroscopic Explorer (FUSE) data are used toinvestigate the molecular hydrogen (H2) content ofintermediate-velocity clouds (IVCs) in the lower halo of the Milky Way.We analyze interstellar absorption toward 56 (mostly extragalactic)background sources to study H2 absorption in the Lyman andWerner bands in 61 IVC components at H I column densities>=1019 cm-2. For data with good signal-to-noiseratio (S/N) (~9 per resolution element and higher), H2 in IVCgas is convincingly detected in 14 cases at column densities varyingbetween ~1014 and ~1017 cm-2. We findan additional 17 possible H2 detections in IVCs in FUSEspectra with lower S/N. The molecular hydrogen fractions, f, varybetween 10-6 and 10-3, implying a dense, mostlyneutral gas phase that is probably related to the cold neutral medium(CNM) in these clouds. If the H2 stays information-dissociation equilibrium, the CNM in these clouds can becharacterized by compact (D~0.1 pc) filaments with volume densities onthe order of nH~30 cm-3. The relatively highdetection rate of H2 in IVC gas implies that the CNM in theseclouds is ubiquitous. More dense regions with much higher molecularfractions may exist, but it would be difficult to detect them inabsorption because of their small size.
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