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Interstellar Iron and Silicon Depletions in Translucent Sight Lines
We report interstellar Fe II and Si II column densities toward sixtranslucent sight lines (AV>~1) observed with the SpaceTelescope Imaging Spectrograph (STIS). The abundances were determinedfrom the absorption of Si II] at 2335 Å, and several weak Fetransitions including the first reported detections of the λ2234line. We derive an empirical f-value for the Fe II λ2234transition of log(fλ)=-1.54+/-0.05. The observed sight linessample a variety of extinction characteristics as indicated by theirRV values, which range from 2.6 to 5.8. The dust-phaseabundances of both Si and Fe are positively correlated with thesmall-grain population (effective radii smaller than a few hundredμm) toward the targets. The physical conditions along the sight linessuggest that this relationship may be due to differences in the survivalof small particles in some interstellar environments. The chemicalcomposition of the small grains could either resemble dust mantles or besilicate-rich.Based on observations 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.
| 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.
| A search for O VI in the winds of B-type stars
We have conducted a survey of FUSE spectra of 235 Galactic B-type starsin order to determine the boundaries in the H-R diagram for theproduction of the superion {O Vi} in their winds. By comparing thelocations and morphology of otherwise unidentified absorption featuresin the vicinity of the {O Vi} resonance doublet with the bona fide windprofiles seen in archival IUE spectra of the resonance lines of {N V},{Si Iv} and {C Iv}, we were able to detect blueshifted {O Vi} lines inthe spectra of giant and supergiant stars with temperature classes aslate as B1. No features attributable to {O Vi} were detected in dwarfslater than B0, or in stars of any luminosity class later than B1,although our ability to recognize weak absorption features in thesestars is severely restricted by blending with photospheric andinterstellar features. We discuss evidence that the ratio of the ionfractions of {O Vi} and {N V} is substantially different in the winds ofearly B-type stars than O-type stars.
| 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.
| 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.
| Unusually Weak Diffuse Interstellar Bands toward HD 62542
As part of an extensive survey of diffuse interstellar bands (DIBs), wehave obtained optical spectra of the moderately reddened B5 V star HD62542, which is known to have an unusual UV extinction curve of the typeusually identified with dark clouds. The typically strongest of thecommonly cataloged DIBs covered by the spectra-those at 5780, 5797,6270, 6284, and 6614 Å-are essentially absent in this line ofsight, in marked contrast with other lines of sight of similarreddening. We compare the HD 62542 line of sight with others exhibitinga range of extinction properties and molecular abundances and interpretthe weakness of the DIBs as an extreme case of deficient DIB formationin a dense cloud whose more diffuse outer layers have been strippedaway. We comment on the challenges these observations pose foridentifying the carriers of the diffuse bands.
| A New Measurement of the Average Far-Ultraviolet Extinction Curve
We have measured the extinction curve in the far-ultraviolet wavelengthregion of (900-1200 Å) using spectra obtained with the BerkeleyEUV/FUV spectrometer during the ORFEUS I and the ORFEUS II (Orbiting andRetrievable Far and Extreme Ultraviolet Spectrometer) missions in 1993and 1996. From the complete sample of early-type stars observed duringthese missions, we have selected pairs of stars with the same spectraltype but different reddenings to measure the differential FUVextinction. We model the effects of molecular hydrogen absorption andexclude affected regions of the spectrum to determine the extinctionfrom dust alone. We minimize errors from inaccuracies in the catalogedspectral types of the stars by making our own determinations of spectraltypes based on their IUE spectra. We find substantial scatter in thecurves of individual star pairs and present a detailed examination ofthe uncertainties and their effects on each extinction curve. We findthat, given the potentially large uncertainties inherent in using thepair method at FUV wavelengths, a careful analysis of measurementuncertainties is critical to assessing the true dust extinction. Wepresent a new measurement of the average far-ultraviolet extinctioncurve to the Lyman limit; our new measurement is consistent with anextrapolation of the standard extinction curve of Savage & Mathis.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
With the high-resolution spectroscopy now available in the optical andsatellite UV, it is possible to determine the neutral/ionized columndensity ratios for several different elements in a single cloud.Assuming ionization equilibrium for each element, one can make severalindependent determinations of the electron density. For the clouds forwhich such an analysis has been carried out, these different estimatesdisagree by large factors, suggesting that some process (or processes)besides photoionization and radiative recombination might play animportant role in the ionization balance. One candidate process is thecollision of ions with dust grains. Making use of recent workquantifying the abundances of polycyclic aromatic hydrocarbon moleculesand other grains in the interstellar medium, as well as recent modelsfor grain charging, we estimate the grain-assisted ion recombinationrates for several astrophysically important elements. We find that theserates are comparable to the rates for radiative recombination forconditions typical of the cold neutral medium. Including grain-assistedion recombination in the ionization equilibrium analysis leads toincreased consistency in the various electron density estimates for thegas along the line of sight to 23 Orionis. However, not all of thediscrepancies can be eliminated in this way; we speculate on some otherprocesses that might play a role. We also note that grain-assistedrecombination of H+ and He+ leads to significantlylower electron fractions than usually assumed for the cold neutralmedium.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| 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.
| Interstellar Carbon Abundance
We present 10 new measurements of carbon gas phase column density in theinterstellar medium. We have used spectra made with the Goddard HighResolution Spectrograph aboard the Hubble Space Telescope containing theCII 1334.5 Ang and CII* 1335.7 Ang lines. The continuum reconstructionmethod has been used to obtain the carbon column density from theLorentzian damped lines. Extinction curve parameters are determined inselected directions and relation between C/H ratio and extinctionparameters is discussed. A correlation has been found between C/H andthe strength of the 2175 Ang bump. Unlike previous results, we noticethat C/H changes with fractional abundance of molecular hydrogen,f(H_2). The average value of C/H=3.55*10^{-4} for lines of sight withf(H_2)<1*10^{-3} is the same as solar photospheric abundance fromGrevese and Noels (1993) and may represent the real cosmic carbonabundance.
| Abundances and Physical Conditions in the Warm Neutral Medium toward μ Columbae
We present ultraviolet interstellar absorption-line measurements for thesightline toward the O9.5 V star μ Columbae (l=237.3d, b=-27.1d d~400pc, z~180 pc; ~0.06 cm-3) obtained withthe Goddard High Resolution Spectrograph (GHRS) on board the HubbleSpace Telescope. These archival data represent the most complete GHRSinterstellar absorption-line measurements for any line of sight towardan early-type star. The 3.5 km s-1 resolution of theinstrument allows us to accurately derive the gas-phase column densitiesof many important ionic species in the diffuse warm neutral medium,including accounting for saturation effects in the data and forcontamination from ionized gas along this sightline. For thelow-velocity material (-20<~vLSR<~+15 kms-1), we use the apparent column density method to derivecolumn densities. For the individual absorbing components atvLSR~-28.8, +20.1, +31.0, and +41.2 km s-1, weapply component fitting techniques to derive column densities andb-values. We have also used observations of interstellar Lyαabsorption taken with the GHRS intermediate resolution gratings toaccurately derive the H I column density along this sightline. Theresulting interstellar column density, logN(H I)=19.86+/-0.015, is inagreement with other determinations but is significantly more precise.The low-velocity material shows gas-phase abundance patterns similar tothe warm cloud (cloud A) toward the disk star ζ Ophiuchi, while thecomponent at vLSR~+20.1 km s-1 shows gas-phaseabundances similar to those found in warm halo clouds. We find that thevelocity-integrated gas-phase abundances of Zn, P, and S relative to Halong this sightline are indistinguishable from solar system abundances.We discuss the implications of our gas-phase abundance measurements forthe composition of interstellar dust grains. We find a dust-phaseabundance [(Fe+Mg)/Si]d =2.7-3.3 in the low-velocity gas;therefore the dust cannot be composed solely of common silicate grains,but must also include oxides or pure iron grains. The low-velocitymaterial along this sightline is characterized by T~6000-7000 K withne~0.3 cm-3, derived from the ionizationequilibrium of Mg and Ca. The relative ionic column density ratios ofthe intermediate-velocity components at vLSR=+31.0 and +41.2km s-1 show the imprint both of elemental incorporation intograins and (photo)ionization. These clouds have low total hydrogencolumn densities [logN(H)~17.4-17.7], and our component fitting b-valuesconstrain the temperature in the highest velocity component to beT=4000+/-700 K. The electron density of this cloud is ne~0.6cm-3, derived from the 2P1/2 to2P3/2 fine structure excitation of C II. Thecomponents at vLSR~-30 and -48 km s-1 along thissightline likely trace shocked gas with very low hydrogen columndensities. The vLSR~-30 km s-1 component isdetected in a few strong low-ionization lines, while both are easilydetected in Si III. The relative column densities of the -30 kms-1 suggest that the gas is collisionally ionized at moderatetemperatures (T~25,000 K). This is consistent with the measured b-valuesof this component, though nonthermal motions likely contributesignificantly to the observed breadths.
| Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud
We have analyzed both high-resolution optical absorption-line spectraand UV spectra obtained with IUE of the LMC SN 1987A, in order todetermine abundances and physical conditions in the various neutralinterstellar clouds along the line of sight to the supernova (SN). Wehave used a flat-fielding procedure to enhance the signal-to-noiseratios (S/Ns) and the reliability of weak features in the UV spectra andhave modeled the UV line profiles using the component structure derivedfrom the higher resolution, high-S/N optical spectra of Ca II and Na I.Fits to the Ca II, Ca I, and Na I absorption-line profiles reveal (atleast) 46 components, at velocities -24 km s^-1<~v_solar<~296 kms^-1, which can be associated with the 10 component groups discerniblein the lower resolution UV spectra. From the UV spectra, we determinedcomponent-group column densities for C I, Mg I, Mg II, Al II, Si II, PII, Cl I, Ti II, Cr II, Mn II, Fe II, Ni II, and Zn II-with 1 sigmauncertainties less than 0.1 dex in many cases. These are the mostextensive and accurate abundances yet measured for the neutral ISM inthe LMC. The component velocities, the patterns of relative elementalabundances [X/Zn] and [X/Fe], and various diagnostic ratios have beenused to estimate the locations and physical characteristics [N(H), T,n_e] of these component groups. (Systematic differences among thediagnostic ratios make the derived physical properties somewhatuncertain, however.) The components at low velocities (5 kms^-1<~v<~23 km s^-1) have relative abundances and values for thediagnostic ratios very similar to those found for warm, diffuse Galacticdisk clouds and likely are due to a mixture of warm and cool gas in theGalactic disk. The components at velocities 56 km s^-1<~v<~90 kms^-1 are due to a mixture of warm and cool gas, apparently withnegligible depletions, in the Galactic halo. The twointermediate-velocity component groups (109 km s^-1<~v<~140 kms^-1 and 155 km s^-1<~v<~176 km s^-1) both have relativeabundances similar to those found for Galactic halo clouds. These warm(T>~4500 K), partially ionized clouds are probably located in theGalactic halo and in the LMC, respectively. The components at velocities191 km s^-1<~v<~225 km s^-1 also have relative abundances similarto those in the halo clouds but are likely due to gas in the LMC,perhaps very close to the SN. The component groups at 238 kms^-1<~v<~255 km s^-1 and 265 km s^-1<~v<~270 km s^-1 areprobably located on opposite sides of the main LMC component group (atvelocities 275 km s^-1<~v<~296 km s^-1) (using absorption-linedata for several other adjacent lines of sight and the structureinferred from SN light-echo observations). Although the relativeabundances and diagnostic ratios for those three LMC groups are similarto those found for warm, low-density Galactic disk clouds, the widths ofindividual components seen in very high resolution spectra of Na I and KI imply that T is generally less than about 1500 K. Higher N(Na I)/N(CaII) ratios, the presence of CH, and the C I fine structure levelpopulations suggest that the main LMC group contains both cool and warmgas. For the LMC components, the total N(H) estimated from the observedrelative abundances and inferred depletions is consistent with the valueobtained from Lyalpha absorption toward the neighboring star Sk -69deg203, after accounting for differences in reddening and for an overallsubsolar metallicity of 0.2-0.3 dex for the LMC ISM. Since the relativeabundance patterns determined for stars and gaseous nebulae in both theSMC and the LMC appear to be similar to the solar pattern (for theelements whose interstellar abundances we have considered), thesimilarities in relative gas-phase interstellar abundances in our Galaxyand in the Magellanic Clouds suggest that the dust depletions followsimilar patterns as well-despite differences in metallicity anddust-to-gas ratio among the three galaxies. These local relativeabundance/depletion patterns may thus be used to infer total (gas+dust)abundances for QSO absorption-line systems at various redshifts.
| Echelle Spectroscopy of Interstellar Absorption toward MU Columbae with the Goddard High Resolution Spectrograph
Goddard High Resolution Spectrograph echelle-mode observations of theinterstellar absorption lines of Mg II, Si IV, C IV, and N V toward muColumbae (HD 38666) are presented. The observations have a spectralresolution of 3.5 km s^-1 and signal-to-noise ratios (S/Ns) of 20-200.The mu Col sight line (l=237.3d, b=-27.1d, d=0.40 kpc, z=-0.18 kpc)extends though the Local Bubble and the warm neutral, warm ionized, andhot ionized phases of the interstellar medium (ISM). The high-ionizationcolumn densities toward mu Col are log N(Si IV)=12.16+/-0.05, log N(CIV)=12.88+/-0.02, and logN(N V)=11.8-12.3. Profile fits to Copernicussatellite measures of O VI absorption toward mu Col yield log N(OVI)=13.82+/-0.01 and b=38.7 km s^-1. This implies N(C IV)/N(OVI)=0.11+/-0.01, which is typical of the values found for the hot ISM ofthe Galactic disk. The O VI profile is twice as broad as the C IV and NV profiles, even though these species have roughly similar averagevelocities. Some of the C IV, N V, and O VI absorption toward mu Col mayoccur at the interface of the Local Cloud and Local Bubble, althoughadditional contributions to these ions probably also occur in moredistant gas along the sight line. A substantial part of the Si IVabsorption likely arises in warm photoionized gas in an H II regionsurrounding mu Col. The profile width differences among thehigh-ionization lines of C IV, N V, and O VI could be produced if theline of sight passes through a highly evolved supernova remnant. Theobservations for mu Col and for other stars observed at high resolutionwith the GHRS reveal that multiple gas types (warm and hot) contributeto the absorption by the highly ionized atoms along both nearby anddistant sight lines. Disentangling the relative contributions from thedifferent gas types requires high-resolution and high-S/N observations.The Mg II observations, combined with a solar Mg reference abundance,imply that the Mg depletion toward mu Col is -0.31 dex. As observed forother sight lines through the warm neutral medium, the gas-phaseobservations of Mg, when combined with results for Fe and Si, suggestthat Mg and Fe are more deficient from the gas phase than one wouldexpect if these elements are only contained in silicate dust grains.
| Interstellar Abundances in the Magellanic Clouds. I. GHRS Observations of the Small Magellanic Cloud Star SK 108
We present HST GHRS echelle-B and G160M spectra of the Wolf-Rayet binarySk 108, located in the northeastern part of the main "bar" of the SmallMagellanic Cloud. The spectra show interstellar absorption from C I, OI, Si II, Si II*, S II, P II, Cr II, Mn II, Fe II, Ni II, and Zn II. Wefind at least 25 interstellar components, which may be grouped viasimilar kinematics and/or similar relative heavy element abundances intotwo sets arising in the Galactic disk and halo, plus three sets locatedin the SMC. The SMC component groups may correspond to large-scale SMCgas complexes identified in H I 21 cm emission surveys. The relative gasphase abundances for Si II, Cr II, Mn II, Fe II, Ni II, and Zn II foundfor the SMC interstellar components are similar to those found forinterstellar clouds in the Galactic halo. Since the relative totalabundances for those elements found for F-G supergiants and gaseousnebulae in the SMC are similar to those present in comparable Galacticobjects, we conclude that the interstellar depletion patterns are alsosimilar in the SMC and in the Galactic halo clouds, despite thesignificantly lower metallicity and dust-to-gas ratio in the SMC. We donot see the distinctive depletion patterns commonly found for cold andwarm clouds in the Galactic disk for any of the SMC components along theline of sight to Sk 108. We discuss some implications of these resultsfor understanding the abundances found for QSO absorption-line systemsand conclude that both nucleosynthetic and depletion effects contributeto the observed abundances--to different degrees for different systems.
| The Abundance of Mg in the Interstellar Medium
An empirical determination of the f-values of the far-UV Mg II lambdalambda 1239, 1240 lines is reported. The strong near-UV Mg II lambdalambda 2796, 2803 lines are generally highly saturated along mostinterstellar sight lines outside the local interstellar medium (ISM) andusually yield extremely uncertain estimates of Mg+ column densities ininterstellar gas. Since Mg+ is the dominant form of Mg in the neutralISM, and since Mg is expected to be a significant constituent ofinterstellar dust grains, the far-UV lines are critical for assessingthe role of this important element in the ISM. This study consists ofcomplete component analyses of the absorption along the lines of sighttoward HD 93521 in the Galactic halo and xi Persei and zeta Ophiuchi inthe Galactic disk, including all four UV Mg+ lines and numerous othertransitions. The three analyses yield consistent determinations of thelambda lambda 1239, 1240 f-values, with weighted means of (6.4 +/- 0.4)x 10-4 and (3.2 +/- 0.2) x 10-4, respectively. These results are afactor of ~2.4 larger than a commonly used theoretical estimate, and afactor of ~2 smaller than a recently suggested empirical revision. Theeffects of this result on gas- and dust-phase abundance measurements ofMg are discussed.
| Absorption by Highly Ionized Interstellar Gas Along Extragalactic and Galactic Sight Lines
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113.2158S&db_key=AST
| High-Resolution Ultraviolet Observations of the Highly Ionized Interstellar Gas toward Radio Loops I and IV
We present new Goddard High-Resolution Spectrograph (GHRS) echelleobservations of the high ionization lines of Si IV, C IV, and N V towardHD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernovaremnants. Absorption along the path to HD 119608 makes it possible tostudy energetic processes that may result in the flow of gas into theGalactic halo. The data have a resolution (FWHM) of ~3.5 km s-1 and S/Nratios of 30:1--50:1. The integrated high ion column densities log N =13.57 +/- 0.02, 14.48 +/- 0.06, and 13.45 +/- 0.07 for Si IV, C IV, andN V, respectively, imply a factor of 2--4 enhancement in the amount ofhighly ionized gas compared to average sight lines through the Galacticdisk and halo. The integrated high ion column density ratios, N(CIV)/N(Si IV) = 8.1 +/- 1.1 and N(C IV)/N(N V) = 10.7 +/- 2.1, are alsoseveral times larger than normal. These high ion results suggest theabsorption is influenced by passage of the sight line through the centerof Loop IV. The HD 119608 C IV absorption profile has a bimodal velocitystructure indicative of an expanding shell; we tentatively derive anexpansion velocity of 16 km s-1 for Radio Loop IV. A detailed analysisof the high ion profile structure indicates that multiple types ofhighly ionized gas with a range of properties exist along this sightline. We also reexamine the high ionization properties of the QSO 3C 273sight line using new intermediate-resolution (FWHM ~ 20 km s-1) GHRSdata. We obtain log N = 14.49 +/- 0.03 and 13.87 +/- 0.06 for C IV and NV, respectively. The C IV column density, which is 0.12 dex smaller thanearlier estimates, leads to somewhat smaller ionic ratios thanpreviously determined. We find N(C IV)/N(Si IV) = 5.1 +/- 0.6 and N(CIV)/N(N V) = 4.2 +/- 0.6. However, as for HD 119608, the high ion columndensities toward 3C 273 are larger than normal by factors of 2--4. The3C 273 high ion absorption profiles are much broader than those seentoward HD 119608 and other sight lines near the center of Loop IV. Thelarger line widths may result because the sight line passes through theturbulent edge of Loop IV as well as the X-ray and radio continuumemission regions of the North Polar Spur. We have compiled a list of thehighest quality IUE and GHRS high ion measurements available forinterstellar sight lines through the disk and halo and find thefollowing median averaged results: N(C IV)/N(Si IV) = 3.8 +/- 1.9 andN(C IV)/N(N V) = 4.0 +/- 2.4. These ratios are lower than those foundfor four Loop IV sight lines. We suggest a model for the production ofhighly ionized gas in Loop IV in which the contributions from turbulentmixing layers and conductive interfaces/SNR bubbles to the total highion column densities are approximately equal. Much of the high ionabsorption toward HD 119608 and 3C 273 may occur within a highlyfragmented medium within the remnant or the outer cavity walls of theremnant.
| Composition of Interstellar Clouds in the Disk and Halo. IV. HD 215733
In this paper we continue our investigation of diffuse clouds in theinterstellar medium with an analysis of the line of sight toward thestar HD 215733, located in the Galactic halo some 1700 pc below theplane. As in our previous papers, we utilize the component-fittingtechnique to determine velocities, velocity widths, and column densitiesfor a variety of ions in each of the absorbing regions detected. Ourdata include a large number of ultraviolet absorption lines observedwith the Goddard High Resolution Spectrograph and ground-basedobservations of Ca II K absorption and H I 21 cm emission. We detect 23components (absorbing regions) in the low-ionization species toward HD215733 and seven components in the highly ionized species (i.e., Si3+,C3+, and N4+). The low-ion components arise in H I gas. Gas-phaseabundances measured for these components follow the pattern seen forhalo stars in our previous studies. These have been interpreted asindicating either that a nearly indestructible population ofinterstellar grains is present or that the intrinsic abundances of anumber of elements in the ISM are significantly subsolar. Kinetictemperatures are estimated for 16 low-ion components. Of these, four(with the highest values of |vLSR|) are warm, with T > 1000 K; sixare cold, with T < 300 K. Extensive diagnostic information includesdata on the excited atoms C+ and C0, showing that in the cold cloudsn(H0) ~= 25 cm-3, with n(H0)T ~= 2500 K cm-3. The ionization equilibriumof C0, Mg0, S0, and Ca+ gives log ne values differing systematically byup to 1 dex between these different species. Correction for thesedifferences, together with a somewhat uncertain overall calibration withexcited C+, gives values of log ne in the range from -2.1 to -2.7 forthe cold clouds in the four best determined cases, and from -1.2 to -1.8for the 3 warm clouds with measured ne. The cold cloud values yieldne/n(H0) ~= 2 x 10-4, which suggests ionization of the heavier elementsonly (with all H neutral), but values higher by half a dex would also beconsistent with the data. Strong absorption features of Si3+ and C3+both appear in three components. For two of these, the ratio of b valuesbetween these two species equals the square root of the mass ratio,suggesting thermal broadening at temperatures of 6 x 105 K and 5 x 104K. The column density ratios would require a temperature of ~8 x 104 Kin collisional equilibrium.
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| The Composition of the Diffuse Interstellar Medium
Recent Goddard High Resolution Spectrograph measurements of Si, S, Cr,Mn, Fe, and Zn in interstellar clouds along lines of sight in theGalactic disk and into the lower halo are discussed. The gas-phaseabundance of S relative to H in the interstellar clouds appears to beindistinguishable from the solar value. For the other elements, we findwell-defined upper limits in the gas-phase abundances at significantlysubsolar values. For Fe, Mn, and Cr (and probably Ti), there are noconvincing cases in which the relative gas-phase abundances exceedroughly -0.5 dex, i.e., these elements are not seen in interstellar gaswith an abundance greater than about one-third solar. For Si, the limitis roughly -0.15 dex, and for Zn a constant abundance of -0.13 dex isfound from seven clouds along one halo sight line. These subsolarmaximum abundances have two possible interpretations: (1) they indicatethe presence of an essentially indestructible component of interstellardust, which contains about two-thirds of the Ti, Mn, Cr, and Fe andabout one-third of the Si (based on solar composition), or (2) theyindicate that the true total abundances of these elements aresubstantially less than in the Sun.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| An HST study of galactic inerstellar zinc and chromium
We present a survey of interstellar Zn II and Cr II absorption extractedfrom the Hubble Space Telescope Goddard High Resolution Spectrograph(HST GHRS) data archive. We find clear evidence for an enhanceddepletion of Zn from the gas phase with increasing fractional abundanceof molecular hydrogen f(H2). Our lower limit to the Galacticinterstellar metallicity is approximately 65% of the solar value asdetermined by the measured Zn abundances in the lowest f(H2) sightlines,(N(Zn)/N(H0tot) = -0.19 +/- 0.04. Thecorrespondingly high depletion of Cr with respect to solar(N(Cr/N(H0tot) = -1.44 +/- 0.26 indicates thatthere are significant amounts of dust present in these lines of sight.The Galactic abundances of Zn and Cr in the ISM provide a fundamentalreference point which is used to understand the metal enrichment anddust formation history of damped Lyman alpha QSO absorption-linesystems, normally believed to arise from intervening precursors tomodern disk galaxies. Although the spread in Zn abundances is large forboth the local ISM and in damped Lyman alpha systems, we still find asubstantial difference (factor of 4-10) in metallicity between the twosets. This survey and future observations of more distant objects whichprobe the full extent of the Milky Way halo provide a more completepicture of the enrichment and depletion characteristics of present-daygalaxies.
| Observations of Interstellar Clouds in the Galactic Halo
The vertical structure of the ISM in the Milky Way is complex. Across-sectional cut of the Galaxy would reveal a stratified distributionof material ranging from the cold (T =~ 10 K) dense molecular clouds,which are closely confined to the Galactic disk, to very hot (T =~ 10(6)K) diffuse gas extending into the Galactic halo with a scale height (h)of perhaps some 5 kpc. Between these extremes lie cold H I (theso-called ``standard clouds''; T =~ 100 K, h =~ 100 pc), warm H I clouds(T =~ 10(3-4) K, h =~ 500 pc), warm ionized H (T =~ 10(3-4) K, h =~ 1kpc), and hot gas (T =~ 10(5) K,h= 2--5 kpc). In order to understand thedynamical processes which produce and maintain this vertical structure--- and to understand the interrelationships among these distinctclasses of interstellar gas --- we must first investigate the physicalproperties of the observed phases of the ISM. Accurate measurements oftemperatures, compositions, ionization states, densities, etc., forindividual interstellar clouds are required for the development of aunified model of the ISM. The UV spectral region is rich inabsorption-line diagnostics which allow the study of interstellar matterover a wide range of physical conditions --- from the cold H I clouds,which produce strong absorption from many singly-ionized metals, to thehot 10(5) K gas seen in absorption from such ions as C(3+) and N(4+) .In this poster I will present results from a multiyear program of UVspectroscopy with the Goddard High Resolution Spectrograph aboard theHubble Space Telescope whose goal is to help characterise the propertiesof individual interstellar clouds in the Galactic disk and halo. I willconcentrate on results for three lines-of-sight extending into the lowerhalo towards the early-type, high-latitude stars HD 93521, HD 149881, HD215733 (z-distances =~ 1500 pc) and discuss the physical conditions inthe interstellar clouds and the implications for understanding theorigin of the structure of the ISM in the Galatic halo.
| Ultraviolet FeIII lines in the spectra of high galactic latitude early-type stars
Using high resolution spectral data from the International UltravioletExplorer satellite, we present qualitative and quantitative comparisonsof blends of Fe III absorption lines in the region1890A<=λ<=1930A for a sample of fifteen high latitudeB-type and standard stars. Standard and halo stars were matched ineffective temperature and surface gravity using Stroemgren [c_1_] andHβ photometry, and LTE model atmosphere codes were used toinvestigate whether they had similar iron abundances. We conclude thatwhile most of the halo stars have Population I iron abundances and maybe young objects, one star, HD 177566, has significant iron and silicondepletions and is most probably an old, evolved star. In view of thecoincidence of the atmospheric parameters of this star with those ofyoung B-type stars, we consider a post-AGB evolutionary status to belikely for HD 177566.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| Infrared Observations of Possible Hot Post-Asymptotic Giant Branch Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...272..243C
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
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