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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.
| 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.
| 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.
| Composition of the Interstellar Medium
We present an analysis of the FeII, GeII, MgI, MgII, SI, SII, SiII andZnII interstellar lines for 63 stars. Column density of MgII, SII, SiIIand ZnII is well correlated with distance. However, the column density-- distance relation should be used with care for the estimation of thedistance to OB stars. For stars with large f(H_2) this relation can leadto a large overestimation of the distance. The column densities of MgII,SiII and GeII (our largest samples of data) normalized to their totalhydrogen column densities decrease with interstellar reddening E(B-V) asexpected for elements that are incorporated into dust grains. The GeIIabundance (GeII/H) is lower in dense molecular clouds. The abundances ofall analyzed elements are generally lower than their Solar Systemvalues.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| New runaway OB stars with HIPPARCOS
A Monte Carlo method for detection of runaway OB stars fromobservational data is proposed. 61 runaway OB star candidates have beendetected by an analysis of Hipparcos proper motions. The peculiartangential and total transverse velocities have been determined forthese stars. A list of the detected runaway star candidates ispresented. The evidence of a discrepancy between photometric andparallactic distances of runaway OB star candidates is presented.
| C II Radiative Cooling of the Diffuse Gas in the Milky Way
The heating and cooling of the interstellar medium (ISM) allow the gasin the ISM to coexist at very different temperatures in thermal pressureequilibrium. The rate at which the gas cools or heats is therefore afundamental ingredient for any theory of the ISM. The heating cannot bedirectly determined, but the cooling can be inferred from observationsof .CII*, which is an important coolant in differentenvironments. The amount of cooling can be measured through either theintensity of the 157.7 μm [C II] emission line or the CII*absorption lines at 1037.018 and 1335.708 Å, observable with theFar Ultraviolet Spectroscopic Explorer and the Space Telescope ImagingSpectrograph on board the Hubble Space Telescope, respectively. Wepresent the results of a survey of these far-UV absorption lines in 43objects situated at |b|>~30deg. Measured column densitiesof CII*, S II, P II, and Fe II are combined with H I 21 cmemission measurements to derive the cooling rates (per H atom using H Iand per nucleon using S II) and to analyze the ionization structure,depletion, and metallicity content of the low-, intermediate-, andhigh-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. Based on the depletion and the ionization structure, the LVCs,IVCs, and HVCs consist mostly of warm neutral and ionized clouds. Forthe LVCs, the mean cooling rate in ergs s-1 per H atom is-25.70+0.19-0.36 dex (1 σ dispersion). Witha smaller sample and a bias toward high H I column density, the coolingrate per nucleon is similar. The corresponding total Galactic C IIluminosity in the 157.7 μm emission line isL~2.6×107 Lsolar. CombiningN(CII*) with the intensity of Hα emission, we derivethat ~50% of the CII* radiative cooling comes from the warmionized medium (WIM). The large dispersion in the cooling rates iscertainly due to a combination of differences in the ionizationfraction, in the dust-to-gas fraction, and physical conditions betweensight lines. For the IVC Intermediate-Velocity (IV) Arch at z~1 kpc wefind that on average the cooling is a factor of 2 lower than in the LVCsthat probe gas at lower z. For an HVC (complex C, at z>6 kpc) we findthe much lower rate of -26.99+0.21-0.53 dex,similar to the rates observed in a sample of damped Lyα absorbersystems (DLAs). The fact that in the Milky Way a substantial fraction ofthe C II cooling comes from the WIM implies that this is probably alsotrue in the DLAs. We also derive the electron density, assuming atypical temperature of the warm gas of 6000 K: for the LVCs,=0.08+/-0.04 cm-3, and for the IV Arch,=0.03+/-0.01 cm-3 (1 σdispersion). Finally, we measured the column densities N(S II) and N(PII) in many sight lines and confirm that sulphur appears undepleted inthe ISM. Phosphorus becomes progressively more deficient whenlogN(HI)>19.7 dex, which can mean that either P becomes more depletedinto dust as more neutral gas is present or P is always depleted byabout -0.3 dex, but the higher value of P II at lower H I column densityindicates the need for an ionization correction.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. I. Composition and Spectral Features
The existence of faint blue stars far above the Galactic plane that havespectra that are similar to nearby Population I B stars presents severalinteresting questions. Among them are the following: Can a Population IB star travel from the disk to a position many kiloparsecs above theplane in a relatively short main-sequence lifetime? Is it possible thatsingle massive star formation is occurring far from the Galactic plane?Are these objects something else masquerading as main-sequence B stars?This paper (the first of two) analyzes the abundances of a sample ofthese stars and reveals several that are chemically similar to nearbyPopulation I B stars, whereas others clearly have abundance patternsmore like those expected in blue horizontal-branch (BHB) orpost-asymptotic giant branch stars. Several of those with old evolvedstar abundances also have interesting features of note in their spectra.We also consider why this sample does not have any classical Be starsand identify at least two nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory operated by the University of Texas at Austin.
| Young Stars far from the Galactic Plane: Runaways from Clusters
Quite recently, a significant number of OB stars far from the galacticplane have been found, situated at z-distances ranging from severalhundreds of pc to several kpc. The short lifetimes of these stars poseproblems for their interpretation in terms of the standard picture ofstar formation. Different mechanisms have been put forward to explainthe existence of these stars, either within the conventional view, orpostulating star formation in the galactic halo itself. These mechanismsrange from arguing that they are misidentified evolved or abnormalstars, to postulating powerful ejection mechanisms for young disk stars;in situ formation also admits several variants. We have collected fromthe literature a list of young stars far from the plane, for which theevidence of youth seems convincing. We discuss two possible formationmechanisms for these stars: ejection from the plane as the result ofdynamical evolution of small clusters (Poveda et al. 1967) and in situformation, via induced shocks created by spiral density waves (Martos etal. 1999). We compute galactic orbits for these stars, and identify thestars that could be explained by one or the other mechanism. We findthat about 90 percent of the stars can be accounted for by the clusterejection mechanism, that is, they can be regarded as runaway stars inthe galactic halo.
| 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.
| 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.
| 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.
| ORFEUS II Echelle Observations of Molecular Hydrogen in the Galactic Halo
The far-ultraviolet spectra of HD 219188, HD 94473, and HD 18100, threeearly-type stars in the Galactic halo, were obtained with the echellespectrometer on the ORFEUS II telescope in 1996 November. We deriveH2 column densities for the rotational levels up toJ''=6 and estimate excitation temperatures and H2dissociation rates for the clouds toward HD 219188 and HD 94473. Theresults indicate that strong radiation fields might exist near theseclouds, but these radiation fields do not seem to act as major heatingsources. No significant H2 absorption is seen toward HD18100. The correlation between our H2 column densities andpublished extinction maps confirms that the molecular clouds areassociated with dust clouds that lie between the observed stars and theSun. We estimate the conversion factor between N(H2) andW(12CO) to be XCO=4+/-2x1019cm-2 (K km s-1)-1 for the clouds alongthe line of sight to HD 94473, consistent with previous estimates towithin a factor of a few.
| 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.
| Dust in the Ionized Medium of the Galaxy: GHRS Measurements of AL III and S III
We present interstellar absorption-line measurements of the ions S IIIand Al III toward the stars beta^1 Sco, mu Col, xi Per, zeta Oph, rhoLeo, and HD 18100 using archival data from the Goddard High-ResolutionSpectrograph on board the Hubble Space Telescope. The ions Al III and SIII trace heavily depleted and nondepleted elements, respectively, inionized gas along the sight lines to these late-O/early-B stars. We usethe photoionization equilibrium code CLOUDY to derive the ionizationcorrection relating the ratio N(Al III) / N(S III) to the gas-phaseabundance [Al/S]_i [=log (N(Al)/N(S))_i-log(Al/S)_solar] in the ionizedgas. For spectral types considered here, the corrections range from 0.1to 0.3 dex and are independent of the assumed ionization parameter,i.e., the ratio of ionizing photon density to mean electron density.Using the results of these photoionization models, we find [Al/S]_i~-1.0in the ionized gas toward beta^1 Sco, xi Per, and zeta Oph; along thelow-density path toward mu Col we find [Al/S]_i~-0.8. Since S is notdepleted onto grains, these values of [Al/S]_i (~[Al/H]_i) imply thatAl-bearing grains are present in the ionized nebulae around these stars.If the warm ionized medium (WIM) of the Galaxy is photoionized by OBstars, the observations of rho Leo and HD 18100 imply [Al/S]_i=-0.4 to-0.5 in the WIM and thus the presence of dust grains containing Al inthis important phase of the interstellar medium. While photoionizationappears to be the most likely origin of the ionization for Al III and SIII, we cannot rule out confusion from the presence of hot,collisionally ionized gas along the sight lines to beta^1 Sco and HD18100. We find that [Al/S]_i in the ionized gas along the six sightlines is anticorrelated with the electron density and average sight-lineneutral density. The degree of grain destruction in the ionized mediumof the Galaxy is not much higher than in the warm neutral medium. Theexistence of grains in the ionized regions studied here has importantimplications for the thermal balance of these regions.
| ORFEUS II echelle spectra: The scale height of interstellar O VI in the halo
FUV high resolution spectra of 18 stars, particularly chosen to observethe interstellar medium (ISM), were obtained during the secondORFEUS-SPAS free flight space shuttle mission in December 1996. Amongthese were 6 objects with a distance to the galactic plane larger than 1kpc, one SMC and 4 LMC stars. This selction of targets is part of theORFEUS program to explore the galactic halo. As the most importanttracer of the hot gas we analyzed the stronger component of theimportant O vi\ doublet in all our ISM spectra. We found an average N(Ovi)x sin |b| of ~ 3.5 x 10(14) cm(-2) on the lines of sight to the 4 LMCstars. Assuming an exponential distribution of O vi\ we calculated amidplane density n0 of 2.07(+0.26}_{-0.24) x 10(-8) cm(-3)and a scale height h0 of 5.50(+2.37}_{-2.09) kpc.
| Diffuse ionized gas toward beta Canis Majoris
This paper presents the study of the interstellar medium toward betaCMa, a disk sight-line known for its low neutral gas density. This studyuses high and medium resolution HST-GHRS spectra including lines fromthe following species: H i, D i, N i, O i, S ii, S iii, Si ii, Si iii,Si iv, Al ii, Al iii, Fe ii, Mg i, Mg ii, Mn ii, C ii and C iv. The lineof sight to beta CMa (153 pc) is dominated by two ionized regions with avelocity difference of 10 km s(-1) . The ionized regions account formost of the total hydrogen column density, around 2 10(19) cm(-2) , andthe neutral gas represents only 10% of the total gas. The two ionizedclouds display characteristics of the warm diffuse gas detected in thedisk and the halo. Their gas-phase abundances indicate that theirdepletion is low, especially for the more ionized of the two clouds.Special models of photoionization by the two EUV-excess stars beta CMaand epsilon CMa would be needed for a detailed discussion of theionizing mechanisms of the clouds ; their ionization ratios arenevertheless roughly compatible with collisional ionization attemperatures around 20 000 K, substantially higher than the kinetictemperatures derived from the line widths. Their characteristics suggestthat the clouds may be in the process of cooling down and recombiningafter having been shocked and ionized by some violent events, possiblyrelated to the Local Bubble formation. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space TelescopeInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.
| Supplementary southern standards for UBV(RI)c photometry
We present UBV(RI)c photometry for 80 southern red and blue stars foruse as additional standards. The data are tied to the Johnson UBV andCousins (RI)c systems and extend the range of the available stars forcolor equation determination, especially in (U-B) for blue stars and(V-R) and (V-I) for red stars. Comparisons with published data are madeand particularly good agreement is found with Bessell for the red(Gliese) stars.
| The Edinburgh-Cape Blue Object Survey - I. Description of the survey
The Edinburgh-Cape Blue Object Survey is a major survey to discover bluestellar objects brighter than B~18 in the southern sky. It is planned tocover an area of sky of 10000 deg^2 with --b-->30 deg and delta<0deg. The blue stellar objects are selected by automatic techniques fromU and B pairs of UK Schmidt Telescope plates scanned with the COSMOSmeasuring machine. Follow-up photometry and spectroscopy are beingobtained with the SAAO telescopes to classify objects brighter thanB=16.5. This paper describes the survey, the techniques used to extractthe blue stellar objects, the photometric methods and accuracy, thespectroscopic classification, and the limits and completeness of thesurvey.
| 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.
| 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.
| Interstellar Gas-Phase Abundances and Physical Conditions toward Two Distant High-Latitude Halo Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...470..893S&db_key=AST
| UHRF observations of the interstellar medium towards two stars in the Galactic Halo.
We present interstellar Na I and Ca II spectra of the halo stars HD18100 and HD 203664, obtained with the Ultra-High Resolution Facility onthe Anglo-Australian Telescope. These observations have spectralresolutions R=~210,000 and signal-to-noise ratios in the range 20-75. Wefind multiple absorbing components along both sightlines and resolve thevelocity structure within the high velocity cloud toward HD 203664. Wediscuss these spectra and their use in future Hubble Space Telescopestudies of HD 18100 and HD 203664.
| The Gas and Dust Abundances of Diffuse Halo Clouds in the Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..211S&db_key=AST
| Interstellar Abundances from Absorption-Line Observations with the Hubble Space Telescope
The Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble SpaceTelescope (HST) has yielded precision abundance results for a range ofinterstellar environments, including gas in the local medium, in thewarm neutral medium, in cold diffuse clouds, and in distant halo clouds.Through GHRS studies, investigators have determined the abundances ofelements such as C, N, 0, Mg, Si, S, and Fe in individual interstellarclouds. These studies have provided new information about thecomposition of interstellar dust gains, the origin of the Galactichigh-velocity cloud system, and the processes that transport gas betweenthe disk and the halo. Precision measurements of the interstellar D to Hratio and of the abundances of r- and s-process elements have alsoprovided fiducial reference values for cosmological and stellarevolutionary observations and theoretical models.
| Atomic Physics with the Goddard High-Resolution Spectrograph on the Hubble Space Telescope. II. Oscillator Strengths for Singly Ionized Iron
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...452..275C&db_key=AST
| Detection of Hot Gas in the Interstellar Medium
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...450..163H&db_key=AST
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