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Detection in the Interstellar Medium of the Weak [Mg II] Transition at 1398.8 Å
High- and medium-resolution ultraviolet spectra from the Space TelescopeImaging Spectrograph and the Goddard High Resolution Spectrograph wereused to search for the weak electric quadrupole transitions of [Mg II]near 1398.8 Å. This forbidden doublet was detected in eight sightlines. We calculate an empirical f-value of (1.29 ± 0.13) ×10-5 from all detections and discuss prospects of usingthis weak line in future studies of translucent interstellar clouds andin damped Ly? systems.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Atomic and Molecular Carbon as a Tracer of Translucent Clouds
Using archival, high-resolution far-ultraviolet Hubble SpaceTelescope/Space Telescope Imaging Spectrograph spectra of 34 Galactic Oand B stars, we measure C I column densities and compare them withmeasurements from the literature of CO and H2 with regard tounderstanding the presence of translucent clouds along the line ofsight. We find that the CO/H2 and CO/C I ratios provide gooddiscriminators for the presence of translucent material, and bothincrease as a function of molecular fraction, fN =2N(H2)/N(H). We suggest that sightlines with values belowCO/H2 ?10-6 and CO/C I ?1 containmostly diffuse molecular clouds, while those with values above sampleclouds in the transition region between diffuse and dark. Thesediscriminating values are also consistent with the change in slope ofthe CO versus H2 correlation near the column density at whichCO shielding becomes important, as evidenced by the change inphotochemistry regime studied by Sheffer et al. Based on the lack ofcorrelation of the presence of translucent material with traditionalmeasures of extinction, we recommend defining "translucent clouds" basedon the molecular content rather than line-of-sight extinctionproperties.
| The Extension of the Transition Temperature Plasma into the Lower Galactic Halo
Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 starsand 30 extragalactic objects have been assembled to study the extensionsof these species away from the Galactic plane into the Galactic halo. HI and Al III mostly trace the warm neutral and warm ionized medium,respectively, while Si IV, C IV, and O VI trace a combination of warmphotoionized and collisionally ionized plasmas. The much larger objectsample compared to previous studies allows us to consider and correctfor the effects of the sample bias that has affected earlier but smallersurveys of the gas distributions. We find that Si IV and C IV havesimilar exponential scale heights of 3.2(+1.0, -0.6) and 3.6(+1.0,-0.8) kpc. The scale height of O VI is marginally smaller with h =2.6 ± 0.6 kpc. The transition temperature gas is ~3 times moreextended than the warm ionized medium traced by Al III with h =0.90(+0.62, -0.33) kpc and ~12 times more extended than the warmneutral medium traced by H I with h = 0.24 ± 0.06 kpc. There is afactor of 2 decrease in the dispersion of the log of the column densityratios for transition temperature gas for lines of sight in the Galacticdisk compared to extragalactic lines of sight through the entire halo.The observations are compared to the predictions of the various modelsfor the production of the transition temperature gas in the halo. Theappendix presents a revision to the electron scale height of Gaensler etal.'s 2008 study based on electron dispersion measures.
| A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for17 different elements sampled over 243 sight lines in the local part ofour Galaxy reveals that the depletions into solid form (dust grains) areextremely well characterized by trends that employ only three kinds ofparameters. One is an index that describes the overall level ofdepletion applicable to the gas in any particular sight line, and theother two represent linear coefficients that describe how to derive eachelement's depletion from this sight-line parameter. The information fromthis study reveals the relative proportions of different elements thatare incorporated into dust at different stages of grain growth. Anextremely simple scheme is proposed for deriving the dust contents andmetallicities of absorption-line systems that are seen in the spectra ofdistant quasars or the optical afterglows of gamma-ray bursts. Contraryto presently accepted thinking, the elements sulfur and krypton appearto show measurable changes in their depletions as the general levels ofdepletions of other elements increase, although more data are needed toascertain whether or not these findings are truly compelling. Nitrogenappears to show no such increase. The incorporation of oxygen into solidform in the densest gas regions far exceeds the amounts that can takethe form of silicates or metallic oxides; this conclusion is based ondifferential measurements of depletion and thus is unaffected byuncertainties in the solar abundance reference scale.Based in large part on published observations from (1) the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555, (2) theFar Ultraviolet Spectroscopic Explorer (FUSE) mission operated by JohnsHopkins University, supported by NASA contract NAS5-32985, and (3) TheCopernicus satellite, supported by NASA grant NAGW-77 to PrincetonUniversity.
| An Ultraviolet Search for Interstellar CS
High- and medium-resolution ultraviolet spectra from the Space TelescopeImaging Spectrograph (STIS) and the Goddard High Resolution Spectrograph(GHRS) were used to study the diatomic molecule CS through the C-X(0,0)band at 1401 Å. The band was modeled to verify profile shape. Therest wavelength of the C-X band is refined to a value of 1400.88 Åand a 3? lower limit is set on the oscillator strength at 0.14based on equivalent width upper limits of the A-X(0,0) CS Band at 2577Å. The strength of the 1401 Å band is compared to otherinterstellar parameters and implications for CS formation anddestruction are briefly discussed.
| A New FUSE Survey of Interstellar HD
We have used archival FUSE data to complete a survey of interstellar HDin 41 lines of sight with a wide range of extinctions. This follow-up toan earlier survey was made to further assess the utility of HD as acosmological probe; to analyze the HD formation process; and to see whattrends with other interstellar properties were present in the data. Weemployed the curve-of-growth method, supported by line profile fitting,to derive accurate column densities of HD. We find that theN(HD)/2N(H2) ratio is substantially lower than the atomic D/Hratio and conclude that the molecular ratio has no bearing on cosmology,because local processes are responsible for the formation of HD. Basedon correlations with E(B-V), H2, CO, and iron depletion, wefind that HD is formed in the densest portion of the clouds; the slopeof the logN(HD)/logN(H2) correlation is greater than 1.0,caused by the destruction rate of HD declining more slowly than that ofH2; and, as a sidelight, that the depletions are densitydependent.
| A CO J = 1-0 survey of common optical/uv absorption sightlines
Context: Comparison of optical/uv absorption line data withhigh-resolution profiles of mm-wave CO emission provides complementaryinformation on the absorbing gas, as toward ? Oph. Over the pastthirty years a wealth of observations of CO and other molecules inoptical/uv absorption in diffuse clouds has accumulated for which nocomparable CO emission line data exist. Aims: To acquire mm-waveJ=1-0 CO emission line profiles toward a substantial sample ofcommonly-studied optical/uv absorption line targets and to compare withthe properties of the absorbing gas, especially the predicted emissionline strengths. Methods: Using the ARO 12 m telescope, weobserved mm-wavelength J=1-0 CO emission with spectral resolution R ?3× 106 and spatial resolution 1' toward a sample of 110lines of sight previously studied in optical/uv absorption lines of CO,H2, CH, etc. Results: Interstellar CO emission was detected along65 of the 110 lines of sight surveyed and there is a generalsuperabundance of CO emission given the distribution of galacticlatitudes in the survey sample. Much of the emission is optically thickor very intense and must emanate from dark clouds or warm dense gas nearHII regions. Conclusions: Judging from the statisticalsuperabundance of CO emission, seen also in the total line of sightreddening, the OB star optical/uv absorption line targets must bephysically associated with the large quantities of neutral gas whose COemission was detected, in which case they are probably influencing theabsorbing gas by heating and/or photoionizing it. This explains whyCO/H2 and 12CO/13CO ratios differ somewhat betweenuv and mm-wave absorption line studies. Because the lines of sight havebeen preselected to have AV ? 1 mag, relatively little ofthe associated material actually occults the targets, making itdifficult for CO emission line observations to isolate the foregroundgas contribution.Based on observations obtained with the ARO Kitt Peak 12 mtelescope.
| Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
We carried out a comprehensive far-UV survey of 12CO andH2 column densities along diffuse molecular Galactic sightlines. This sample includes new measurements of CO from HST spectraalong 62 sight lines and new measurements of H2 from FUSEdata along 58 sight lines. In addition, high-resolution optical datawere obtained at the McDonald and European Southern Observatories,yielding new abundances for CH, CH+, and CN along 42 sightlines to aid in interpreting the CO results. These new sight lines wereselected according to detectable amounts of CO in their spectra andprovide information on both lower density (<=100 cm-3) andhigher density diffuse clouds. A plot of logN(CO) versuslogN(H2) shows that two power-law relationships are neededfor a good fit of the entire sample, with a break located atlogN(CO,cm-2)=14.1 and logN(H2)=20.4,corresponding to a change in production route for CO in higher densitygas. Similar logarithmic plots among all five diatomic molecules revealadditional examples of dual slopes in the cases of CO versus CH (breakat logN=14.1, 13.0), CH+ versus H2 (13.1, 20.3),and CH+ versus CO (13.2, 14.1). We employ both analytical andnumerical chemical schemes in order to derive details of the molecularenvironments. In the denser gas, where C2 and CN moleculesalso reside, reactions involving C+ and OH are the dominantfactor leading to CO formation via equilibrium chemistry. In thelow-density gas, where equilibrium chemistry studies have failed toreproduce the abundance of CH+, our numerical analysis showsthat nonequilibrium chemistry must be employed for correctly predictingthe abundances of both CH+ and CO.
| Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History
We present an analysis of Kr I λ1236 line measurements from 50sight lines in the Hubble Space Telescope Space Telescope ImagingSpectrograph and Goddard High Resolution Spectrograph data archives thathave sufficiently high resolution and signal-to-noise ratio to permitreliable krypton-to-hydrogen abundance ratio determinations. Thedistribution of Kr/H ratios in this sample is consistent with a singlevalue for the ISM within 5900 pc of the Sun, log10(Kr/H)=-9.02+/-0.02,apart from a rough annulus from between ~600 and 2500 pc distant. TheKr/H ratio toward stars within this annulus is elevated by approximately0.11 dex, similar to previously noted elevations of O/H and Cu/Hgas-phase abundances beyond ~800 pc. A significant drop in the gas-phaseN/O ratio in the same region suggests that this is an artifact ofnucleosynthetic history. Since the physical scale of the annulus' inneredge is comparable to the radius of the Gould Belt and the outer limitof heliocentric distances where the D/H abundance ratio is highlyvariable, these phenomena may be related to the Gould Belt's origins.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555 FUSE is operated for NASA by theJohn Hopkins University under NASA contract NAS-32985.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way
To probe the distribution and physical characteristics of interstellargas at temperatures T~3×105 K in the disk of the MilkyWay, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) toobserve absorption lines of O VI λ1032 toward 148 early-typestars situated at distances >1 kpc. After subtracting off a mildexcess of O VI arising from the Local Bubble, combining our new resultswith earlier surveys of O VI, and eliminating stars that showconspicuous localized X-ray emission, we find an average O VI midplanedensity n0=1.3×10-8 cm-3. Thedensity decreases away from the plane of the Galaxy in a way that isconsistent with an exponential scale height of 3.2 kpc at negativelatitudes or 4.6 kpc at positive latitudes. Average volume densities ofO VI along different sight lines exhibit a dispersion of about 0.26 dex,irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size anddensity, but instead is distributed in regions that have a very broadrange of column densities, with the more strongly absorbing cloudshaving a lower space density. Line widths and centroid velocities aremuch larger than those expected from differential Galactic rotation, butthey are nevertheless correlated with distance and N(O VI), whichreinforces our picture of a diverse population of hot plasma regionsthat are ubiquitous over the entire Galactic disk. The velocity extremesof the O VI profiles show a loose correlation with those of very stronglines of less ionized species, supporting a picture of a turbulent,multiphase medium churned by shock-heated gas from multiple supernovaexplosions.
| The Variation of Magnesium Depletion with Line-of-Sight Conditions
In this paper we report on the gas-phase abundance of singly ionizedmagnesium (Mg II) in 44 lines of sight, using data from the Hubble SpaceTelescope (HST). We measure Mg II column densities by analyzing medium-and high-resolution archival Space Telescope Imaging Spectrograph (STIS)spectra of the 1240 Å doublet of Mg II. We find that Mg IIdepletion is correlated with many line-of-sight parameters [e.g.,f(H2), EB-V, EB-V/r, AV, andAV/r] in addition to the well-known correlation with. These parameters should be more directly relatedto dust content and thus have more physical significance with regard tothe depletion of elements such as magnesium. We examine the significanceof these additional correlations as compared to the known correlationbetween Mg II depletion and . While none of thecorrelations are better predictors of Mg II depletion than, some are statistically significant even assumingfixed . We discuss the ranges over which thesecorrelations are valid, their strength at fixed ,and physical interpretations.
| New Insights on Interstellar Gas-Phase Iron
In this paper we report on the gas-phase abundance of singly ionizediron (Fe II) for 51 lines of sight, using data from FUSE. Fe II columndensities are derived by measuring the equivalent widths of several UVabsorption lines and subsequently fitting those to a curve of growth.Our derivation of Fe II column densities and abundances creates thelargest sample of iron abundances in moderately to highly reddened linesof sight explored with FUSE, lines of sight that are on average morereddened than lines of sight in previous Copernicus studies. We presentthree major results. First, we observe the well-established correlationbetween iron depletion and and also find trendsbetween iron depletion and other line-of-sight parameters [e.g.,f(H2), EB-V, and AV], and we examinethe significance of these trends. Of note, a few of our lines of sightprobe larger densities than previously explored and we do not seesignificantly enhanced depletion effects. Second, we present twodetections of an extremely weak Fe II line at 1901.773 Å in thearchival STIS spectra of two lines of sight (HD 24534 and HD 93222). Wecompare these detections to the column densities derived through FUSEspectra and comment on the line's f-value and utility for future studiesof Fe II. Finally, we present strong anecdotal evidence that the Fe IIf-values derived empirically through FUSE data are more accurate thanprevious values that have been theoretically calculated, with theprobable exception of f1112.
| Hubble Space Telescope Survey of Interstellar 12CO/13CO in the Solar Neighborhood
We examine 20 diffuse and translucent Galactic sight lines and extractthe column densities of the 12CO and 13COisotopologues from their ultraviolet A-X absorption bands detected inarchival Space Telescope Imaging Spectrograph data withλ/Δλ>=46,000. Five more targets with GoddardHigh-Resolution Spectrograph data are added to the sample that more thandoubles the number of sight lines with published Hubble Space Telescopeobservations of 13CO. Most sight lines have12CO-to-13CO isotopic ratios that are notsignificantly different from the local value of 70 for12C/13C, which is based on millimeter-waveobservations of rotational lines in emission from CO and H2COinside dense molecular clouds, as well as on results from opticalmeasurements of CH+. Five of the 25 sight lines are found tobe fractionated toward lower 12C/13C values, whilethree sight lines in the sample are fractionated toward higher ratios,signaling the predominance of either isotopic charge exchange orselective photodissociation, respectively. There are no obvious trendsof the 12CO-to-13CO ratio with physical conditionssuch as gas temperature or density, yet 12CO/13COdoes vary in a complicated manner with the column density of either COisotopologue, owing to varying levels of competition between isotopiccharge exchange and selective photodissociation in the fractionation ofCO. Finally, rotational temperatures of H2 show that allsight lines with detected amounts of 13CO pass through gasthat is on average colder by 20 K than the gas without 13CO.This colder gas is also sampled by CN and C2 molecules, thelatter indicating gas kinetic temperatures of only 28 K, enough tofacilitate an efficient charge exchange reaction that lowers the valueof 12CO/13CO.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made byHST STIS and FUSE to determine the column densities and rotationalexcitation temperatures for carbon monoxide and molecular hydrogen,respectively, along 23 sight lines to Galactic O and B stars. Thereddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse totranslucent interstellar medium (ISM). We find that the H2column densities range from 5×1018 to8×1020 cm-2 and the CO from upper limitsaround 2×1012 cm-2 to detections as high as1.4×1016 cm-2. CO increases with increasingH2, roughly following a power law of factor ~2. TheCO/H2 column density ratio is thus not constant, ranging from10-7 to 10-5, with a mean value of3×10-6. The sample segregates into ``diffuse'' and``translucent'' regimes, the former with molecular fraction <~0.25and AV/d<1 mag kpc-1. The mean CO/H2for these two regimes are 3.6×10-7 and9.3×10-6, respectively, significantly lower than thecanonical dark cloud value of 10-4. Six sight lines show theisotopic variant 13CO, and the isotopic ratio we observe(~50-70) is consistent with, if perhaps a little below, the average12C/13C for the ISM at large. The averageH2 rotational excitation temperature is 74+/-24 K, agreeingwell with previous studies, and the average CO temperature is 4.1 K,with some sight lines showing temperatures as high as 6.4 K. The higherexcitation CO is observed with higher column densities, consistent withthe effects of photon trapping in clouds with densities in the 20-100cm-3 range. We discuss the implications for the structure ofthe diffuse/translucent regimes of the ISM and the estimation ofmolecular mass in galaxies.
| The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Phosphorus in the diffuse interstellar medium
We present FUSE and HST/STIS measurements of the P ii column densitytoward Galactic stars. We analyzed P ii through the profile fitting ofthe unsaturated λ1125 and λ1533 lines and derived columndensities integrated along the sightlines as well as in individualresolved components. We find that phosphorus is not depleted along thosesightlines sampling the diffuse neutral gas. We also investigate thecorrelation existing between P ii and O i column densities and find thatthere is no differential depletion between these two specie.Furthermore, the ratio N(P ii)/N(O i) is consistent with the solar P/Ovalue, implying that P ii and O i coexist in the same gaseous phase andare likely to evolve in parallel. We argue that phosphorus, as traced byP ii, is an excellent neutral oxygen tracer in various physicalenvironments, except when ionization corrections are a significantissue. Thus, P ii lines (observable with FUSE, HST/STIS, or withVLT/UVES for the QSO sightlines) are particularly useful as a proxy forO i lines when these are saturated or blended.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Oxygen Gas-Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygenabundance along the sight lines toward 19 early-type Galactic stars atan average distance of 2.6 kpc. We derive O I column densities fromHubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS)observations of the weak 1355 Å intersystem transition. We derivetotal hydrogen column densities [N(HI)+2N(H2)] using HST/STISobservations of Lyα and Far Ultraviolet Spectroscopic Explorer(FUSE) observations of molecular hydrogen. The molecular hydrogencontent of these sight lines ranges fromf(H2)=2N(H2)/[N(HI)+2N(H2)]=0.03 to0.47. The average of6.3×1021 cm-2 mag-1 with astandard deviation of 15% is consistent with previous surveys. The meanoxygen abundance along these sight lines, which probe a wide range ofGalactic environments in the distant interstellar medium, is106 (O/H)gas=408+/-13 (1 σ in the mean). Wesee no evidence for decreasing gas-phase oxygen abundance withincreasing molecular hydrogen fraction, and the relative constancy of(O/H)gas suggests that the component of dust containing theoxygen is not readily destroyed. We estimate that, if 60% of the dustgrains are resilient against destruction by shocks, the distantinterstellar total oxygen abundance can be reconciliated with the solarvalue derived from the most recent measurements of 106(O/H)gassolar=517+/-58 (1 σ). We note thatthe smaller oxygen abundances derived for the interstellar gas within500 pc or from nearby B star surveys are consistent with a localelemental deficit.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| 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.
| Synthetic High-Resolution Line Spectra of Star-forming Galaxies below 1200 Å
We have generated a set of far-ultraviolet stellar libraries usingspectra of OB and Wolf-Rayet stars in the Galaxy and the Large and SmallMagellanic Cloud. The spectra were collected with the Far UltravioletSpectroscopic Explorer and cover a wavelength range from 1003.1 to1182.7 Å at a resolution of 0.127 Å. The libraries extendfrom the earliest O to late-O and early-B stars for the Magellanic Cloudand Galactic libraries, respectively. Attention is paid to the complexblending of stellar and interstellar lines, which can be significant,especially in models using Galactic stars. The most severe contaminationis due to molecular hydrogen. Using a simple model for the H2line strength, we were able to remove the molecular hydrogen lines in asubset of Magellanic Cloud stars. Variations of the photospheric andwind features of C III λ1176, O VI λλ1032, 1038, PV λλ1118, 1128, and S IV λλ1063, 1073, 1074are discussed as a function of temperature and luminosity class. Thespectral libraries were implemented into the LavalSB and Starburst99packages and used to compute a standard set of synthetic spectra ofstar-forming galaxies. Representative spectra are presented for variousinitial mass functions and star formation histories. The valid parameterspace is confined to the youngest ages of less than ~=10 Myr for aninstantaneous burst, prior to the age when incompleteness of spectraltypes in the libraries sets in. For a continuous burst at solarmetallicity, the parameter space is not limited. The suite of models isuseful for interpreting the rest-frame far-ultraviolet in local andhigh-redshift galaxies. Based on observations made with theNASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operatedfor NASA by the Johns Hopkins University under NASA contract NAS5-32985.
| Origins of the Highly Ionized Gas along the Line of Sight toward HD 116852
We present Hubble Space Telescope Imaging Spectrograph (HST/STIS) andFar Ultraviolet Spectroscopic Explorer (FUSE) observations of high ioninterstellar ultraviolet absorption along the sight line to HD 116852.At a distance of 4.8 kpc, HD 116852 is an O9 III star lying in the lowGalactic halo, -1.3 kpc from the plane of the Galaxy in the directionl=304.9d, b=-16.1d. The sight line passes underneath theSagittarius-Carina and the Norma-Centaurus spiral arms. The STIS E140Hgrating observations provide high-resolution (FWHM~2.7kms-1)spectra of the resonance doublets of Si IV, C IV, and N V. These dataare complemented by medium-resolution (FWHM~20kms-1) FUSEspectra of O VI. The integrated logarithmic column densities are logN(SiIV)=13.60+/-0.02, logN(C IV)=14.08+/-0.03, logN(NV)=13.34+0.05-0.06, and logN(O VI)=14.28+/-0.01.We find evidence for three distinct types of highly ionized gas presentin the data. First, two narrow absorption components are resolved in theSi IV and C IV profiles, at approximate LSR velocities of -36 and -10 kms-1 . These narrow components appear to be produced in gasassociated with the Norma and Sagittarius spiral arms, at approximatez-distances of -1.0 and -0.5 kpc, respectively. The temperature of thegas in these narrow components, as implied by their b-values, suggeststhat the gas is photoionized. The ratio of C IV to Si IV in these narrowcomponents is low compared to the Galactic average. Second, we detect anintermediate-width component in C IV and Si IV, at 17 km s-1,which we propose could arise at the conductive interface at the boundarybetween a low column density neutral or weakly ionized cloud and thesurrounding hot medium. Finally, a broad collisionally ionized componentof gas responsible for producing the smooth N V and O VI profiles isobserved; such absorption is also present to a lesser degree in theprofiles of Si IV and C IV. The broad O VI absorption is observed at avelocity displaced from the broad C IV component by almost 20 kms-1, an amount large enough to suggest that the two ions maynot coexist in the same physical location. If these two ions do existtogether, then the ratio N(C IV)/N(O VI) is too low to be consistentwith turbulent mixing layer models, but could be explained by radiativecooling or conductive heating models. Combining our results with highresolution observations of four other sight lines from the literature,we find an average C IV component frequency of1.0+/-0.25kpc-1.
| Far Ultraviolet Spectroscopic Explorer Observations of O VI Overlying the Scutum Supershell
We present Far Ultraviolet Spectroscopic Explorer observations of the B0III star HD 177989 (l=17.8d, b=-11.9d, d=4.9 kpc). This line of sightpasses through the high-latitude outflow from the Scutum supershell (GS018-04+44), a structure that lies at a kinematic distance of ~3.5 kpcand spans ~5° in diameter. The O VI λ1031.93 line is comparedwith Space Telescope Imaging Spectrograph (STIS) observations of Si IV,C IV, and N V to examine the ionizing mechanisms responsible forproducing the high ion absorption, as well as to study the processes bywhich gas is expelled into the halo. The O VI profile spans a similarvelocity range as the other highly ionized atoms, from -70 to +80 kms-1. Component fits reveal very broad absorption at thekinematic velocity of the Scutum shell, which differs from the narrow SiIV, C IV, and N V components, suggesting that these other species occupya more confined region. It is possible that the O VI is present in hotgas from the shell interior that is diffusing into the high-latituderegion above it. The column densities in the Scutum supershell componentare N(SiIV)=(3.59+/-0.09)×1013,N(CIV)=(1.78+/-0.04)×1014,N(NV)=(8.89+/-0.79)×1012, and N(OVI)=(7.76+/-0.75)×1013 cm-2. The correspondingcolumn density ratios of N(CIV)/N(SiIV)=4.96+/-0.17,N(CIV)/N(NV)=20.0+/-1.8, and N(CIV)/N(OVI)=2.29+/-0.23 show that whilethe C IV and Si IV columns are amplified in this region, the enhancementis not reflected in N V or O VI. We suggest that such ionic ratios andcolumn densities could be produced by ~150-200 turbulent mixing layerslying in a fragmented medium above the polar cap of the superbubble. Wenote through a sight-line comparison that although these absorptioncharacteristics are similar to those near the center of Radio Loops Iand IV, they differ considerably from those near the edges of the loops.It is apparent that the traits of high ion absorption in a supershell,as well as the ionizing mechanisms responsible, can vary among sightlines through the shell.
| IUE Absorption-Line Observations of the Moderately and Highly Ionized Interstellar Medium toward 164 Early-Type Stars
We present measurements of Galactic interstellar Al III, Si IV, and C IVabsorption recorded in high-resolution archival ultraviolet spectra of164 hot early-type stars observed by the International UltravioletExplorer (IUE) satellite. The objects studied were drawn from the listof hot stars scheduled to be observed with the Far UltravioletSpectroscopic Explorer (FUSE) satellite as part of observing programsdesigned to investigate absorption by O VI in the Galactic disk andhalo. Multiple IUE echelle-mode integrations have been combined toproduce a single ultraviolet (1150-1900 Å) spectrum of each starwith a spectral resolution of ~25 km s-1 (FWHM). Selectedabsorption-line profiles are presented for each star along with plots ofthe apparent column density per unit velocity for each line of the AlIII, Si IV, and C IV doublets. We report absorption-line equivalentwidths, absorption velocities, and integrated column densities based onthe apparent optical depth method of examining interstellar absorptionlines. We also determine column densities and Doppler parameters fromsingle-component curve-of-growth analyses. The scientific analysis ofthese observations will be undertaken after the FUSE satellite producessimilar measurements for absorption by interstellar O IV, Fe III, S III,and other ions. Based on archival data from observations obtained withthe International Ultraviolet Explorer (IUE) satellite sponsored byNASA, SERC, and ESA.
| STIS and GHRS Observations of Warm and Hot Gas Overlying the Scutum Supershell (GS 018-04+44)
We present Space Telescope Imaging Spectrograph (STIS) and Goddard HighResolution Spectrograph (GHRS) observations of interstellar UVabsorption toward HD 177989, a B0 III star at 4.9 kpc in the directionl=17.8d, b=-11.9d. The line of sight passes through the high-latitudeejecta of the Scutum supershell (GS 018-06+44), which is ~5° indiameter extending ~7° below the Galactic plane at a kinematicdistance of ~3.5 kpc in the Scutum spiral arm. The observations with theSTIS E140H and GHRS echelle B gratings provide far- and middle-UVspectra at resolutions (FWHM) of ~3 km s-1 and asignal-to-noise ratio (S/N) of ~20:1 to 50:1. The observations revealstrong and broad absorption in the lines of Si IV and C IV centered onLSR velocities of +18 and +42 km s-1 and weaker absorptionfrom these ions near -50 and -13 km s-1. Weak absorption by NV extends over the full velocity range traced by Si IV and C IV. The +18km s-1 high-ionization absorption likely occurs in gas ~400pc below the Sagittarius spiral arm, while the extremely strong +42 kms-1 absorption occurs in highly ionized gas in the Scutumsupershell at a distance of ~700 pc below the Galactic plane. Theproperties of the highly ionized gas associated with the Scutumsupershell are similar to the gas found in radio loops I and IV; in bothcases there is a strong enhancement in the column density of C IVwithout a corresponding increase in the column density of N V, whichcauses N(C IV)/N(N V) to be among the largest measured in theinterstellar medium. The low-ionization absorption lines of N I, S II,Si II, and Fe II produce narrow absorption features at +37, +40 kms-1 and +55, +60 km s-1. The strength andkinematic properties of these absorption features bear no resemblance tothose expected for the high-latitude neutral cloud seen in the H I 21 cmline. This may be due to the relatively low angular resolution (FWHM~21') of the 21 cm observations. The kinematic relationships among thehigh-ionization and low-ionization absorption lines observed in the UVsuggest a related origin in a hot-warm gas interface region. We arepossibly seeing the warm gas in the swept-up shell surrounding a regionwhere hot gas is being vented into the halo. In the warm gas, N(N I)/N(SII) ~0.01 solar, which implies a similar value forN(H0)/[N(H0)+N(H+)]. The warm neutraland ionized gas in the matter overlying the Scutum supershell has valuesof Si /S and Fe /S roughly similar to those found in the warm neutralmedium of the Galactic disk in the vicinity of the Sun. While there hasbeen grain processing in the ejecta of the Scutum supershell, theprocessing has not been complete. Based on observations of interstellarC IV and Si IV at high S/N and high resolution toward four very distantstars, we determine that highly ionized gas absorption components occurat a frequency of ~1 component kpc-1. The strongestcomponents are associated with lines of sight that pass over or underspiral arms or that pass though Galactic supershells. Based onobservations obtained with the Space Telescope Imaging Spectrograph andthe Goddard High Resolution Spectrograph on the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| The Abundance of Interstellar Boron
We use new Space Telescope Imaging Spectrograph (STIS) and archivalGoddard High Resolution Spectrograph (GHRS) observations to studyinterstellar B II λ1362 and O I λ1355 absorption alongseven sight lines. Our new column density measurements, combined withmeasurements of four sight lines from the literature, allow us to studythe relative B/O abundances over a wide range of interstellarenvironments. We measure sight-line-integrated relative gas-phaseabundances in the range [B/O]=-1.00 to -0.17, and our data show that theB/O abundances are anticorrelated with average sight line densities overthe range log~-1.3 to +0.7. Detailed comparisons ofthe B II and O I line shapes show that the B/O ratio is significantlyhigher in warm interstellar clouds than in cool clouds. These resultsare consistent with the incorporation of boron into dust grains in thediffuse ISM. Since boron is likely incorporated into grains, we derive alower limit to the present-day total (gas+dust) interstellar boronabundance of B/H>~(2.5+/-0.9)×10-10. The effects ofdust depletion and ionization differences from element to element willmake it very difficult to reliably determine11B/10B along most interstellar sight lines. Basedon observations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Science Institute. STScI isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS 5-26555.
| Magellanic Cloud-Type Interstellar Dust along Low-Density Sight Lines in the Galaxy
We have studied the UV extinction properties along 30 Galactic sightlines using data from the International Ultraviolet Explorer archivethat have never been previously examined. These distant (d>1 kpc)sight lines were selected to investigate the distribution and physicalconditions of gas located in low-density regions of the Galactic diskand halo. The average densities along these sight lines are extremelylow. It is likely that they are dominated by the warm intercloud mediumand have little contribution from the cold cloud medium. We find that asubsample of these sight lines has extinction curves with weak bumps andvery steep far-UV extinction reminiscent of the Magellanic Clouds. Thesesight lines all lie in the region bounded by325deg<=l<=0deg and-5deg>=b>=-11deg. The gas along these sightlines shows forbidden velocities, which may indicate that the dust hasbeen subject to shocks. This type of low-density sight line may mimicthe environments found in the Magellanic Clouds. Large values of N(CaII)/N(Na I) indicating low depletion are associated with steep far-UVextinction. A possible correlation exists between decreasing bumpstrength and increasing far-UV steepness for extinction curves in theGalaxy and the Magellanic Clouds.
| Observational Evidence of Supershell Blowout in GS 018-04+44: The Scutum Supershell
Emission in the H I 21 cm line has been mapped for a region of theGalaxy that includes two known supershells, GS 018-04+44 and GS034-06+65. We focus on the GS 018-04+44, hereafter referred to as theScutum Supershell, which is an elongated shell about 5 deg in diameterextending to -7 deg below the Galactic plane. The Scutum shell lies at akinematic distance of ~3300 pc, implying a shell diameter of ~290 pcwith a vertical extension of ~400 pc away from the Galactic plane. TheScutum shell contains 6.2x105 Msolar swept intothe walls. We observe that the top of the shell is missing, and asubstantial column of H I rises from the shell walls tob=-11deg, culminating in a large cloud of neutral hydrogen,3.74x104 Msolar, located ~630 pc from the plane.ROSAT data show X-ray emission that closely anticorrelates with the 21cm emission. This emission probably originates from hot gas within theScutum Supershell. After approximately correcting for the foregroundabsorption, we find that the 1.5 keV X-rays peak at the base of theshell, the 0.75 keV emission peaks in the interior and at the top of theshell, and the 0.25 keV emission extends to high latitudes above theshell. The X-ray luminosity is roughly ~5x1036 ergss-1. The Wisconsin Hα Mapper (WHAM) survey shows thepresence of Hα emission that exhibits a morphology similar to thatof the H I. Spectra indicate the presence of ionized hydrogen atvelocities similar to the H I, placing ionized material at the samekinematic distance as the neutral material. IRAS images in the 60 and100 μm wavebands reveal the presence of dust correlated with theneutral hydrogen. Infrared surface brightness indicates an excess in the100 μm emission, which could indicate a molecular hydrogen componentwith a column density of 2.4x1021 cm-2 in thedensest regions of the high-latitude cloud of neutral hydrogen. IUEultraviolet high dispersion spectra of HD 177989 (l=17.89d, b=-11.88d)and HD 175754 (l=16.40d, b=-9.92d) reveal the presence of very strongabsorption by highly ionized gas at a velocity that associates theabsorption with the ejecta of the Scutum Supershell. In the case of HD177989, the high ion column density ratios suggest an origin in aturbulent mixing layer where hot and cool gases mix in the presence ofshear flows. The Hα and X-ray emission suggest that a multitude ofenergetic phenomena exist in this region, providing the necessaryionizing radiation. Indeed, there are multiple supernova remnants, H IIregions, and hot stars, which could all contribute sizeable amounts ofenergy and ionizing radiation. The combination of these data setsindicates observational evidence of a ``blowout'' phenomena whereby hotmaterial produced within the Scutum Supershell has blown through the topof the shell and been pushed to high latitude. Based on observationsfrom the Green Bank 43m radio telescope, the ROSAT All-Sky Survey, theWHAM All-Sky Survey, IRAS, and IUE.
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