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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.
| VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2
Model atmosphere fits to high-resolution optical spectra ofWray 977 confirm the B hypergiant classification ofthe massive companion to the X-ray pulsar GX301-2.The models give a radius of 62 Rȯ, an effectivetemperature of 18 100 K and a luminosity of 5 × 105Lȯ. These values are somewhat reduced compared to thestellar parameters of Wray 977 measured previously.The deduced mass-loss rate and terminal velocity of the stellar wind are10-5 Mȯ yr-1 and 305 kms-1, respectively. The interstellar Na I D absorptionindicates that Wray 977 is located behind the firstintersection with the Sagittarius-Carina spiral arm (1-2.5 kpc) andprobably belongs to the stellar population of the Norma spiral arm at adistance of 3-4 kpc. The luminosity derived from the model atmosphere isconsistent with this distance (3 kpc). The luminosity of the wind-fedX-ray pulsar (LX 1037 erg s-1) isin good accordance with the Bondi-Hoyle mass accretion rate. The spectraobtained with UVES on the Very Large Telescope (VLT) cover a full orbitof the system, including periastron passage, from which we derive theradial-velocity curve of the B hypergiant. The measured radial-velocityamplitude is 10 ± 3 km s-1 yielding a mass ratio q =M_X/M_opt = 0.046 ± 0.014. The absence of an X-ray eclipseresults in a lower limit to the mass of Wray 977 of39 Mȯ. An upper limit of 68 or 53 Mȯ isderived for the mass of Wray 977 adopting a maximumneutron star mass of 3.2 or 2.5 Mȯ, respectively. Thecorresponding lower limit to the system inclination is i > 44°,supporting the view that the dip in the X-ray lightcurve is due toabsorption by the dense stellar wind of Wray 977(Leahy 2002). The "spectroscopic" mass of Wray 977 is43 ± 10 Mȯ, consistent with the range in massderived from the binarity constraints. The mass of the neutron star is1.85 ± 0.6 Mȯ. Time series of spectral linesformed in the dense stellar wind (e.g. He I 5876 Å and H α)indicate the presence of a gas stream trailing the neutron star in itsorbit. The long-term behaviour of the H α equivalent widthexhibits strong variations in wind strength; the sampling of the data isinsufficient to conclude whether a relation exists between windmass-loss rate and pulsar spin period.
| The Interstellar N/O Abundance Ratio: Evidence for Local Infall?
Sensitive measurements of the interstellar gas-phase oxygen abundancehave revealed a slight oxygen deficiency (~15%) toward stars within 500pc of the Sun as compared to more distant sight lines. Recent FUSEobservations of the interstellar gas-phase nitrogen abundance indicatelarger variations, but no trends with distance were reported due to thesignificant measurement uncertainties for many sight lines. Byconsidering only the highest quality (>=5 σ) N/O abundancemeasurements, we find an intriguing trend in the interstellar N/O ratiowith distance. Toward the seven stars within ~500 pc of the Sun, theweighted mean N/O ratio is 0.217 +/- 0.011, while for the six starsfarther away the weighted mean value (N/O = 0.142 +/- 0.008) iscuriously consistent with the current solar value (N/O =0.138+0.20-0.18). It is difficult to imagine ascenario invoking environmental (e.g., dust depletion or ionization)variations alone that explains this abundance anomaly. Is the enhancednitrogen abundance localized to the solar neighborhood or evidence of amore widespread phenomenon? If it is localized, then recent infall oflow-metallicity gas in the solar neighborhood may be the bestexplanation. Otherwise, the N/O variations may be best explained bylarge-scale differences in the interstellar mixing processes for AGBstars and Type II supernovae.Based on data obtained by the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE) mission operated by the Johns HopkinsUniversity. Financial support to US participants has been provided byNASA contract NAS5-32985.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far-ultravioletwavelengths (FUV; 1050-1200 Å) for nine Galactic sight lines. TheFUV extinction was measured using data from the Far UltravioletSpectroscopic Explorer. The sight lines were chosen for their unusualextinction properties in the infrared through the ultraviolet; that theyprobe a wide range of dust environments is evidenced by the large spreadin their measured ratios of total to selective extinction,RV=2.43-3.81. We find that extrapolation of the Fitzpatrick& Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions ofthe FUV extinction based on the Cardelli, Clayton, & Mathis (CCM)dependence on RV give mixed results. For the seven extinctioncurves well represented by CCM in the infrared through ultraviolet(x<8 μm-1), the FUV extinction is well predicted inthree sight lines, overpredicted in two sight lines, and underpredictedin two sight lines. A maximum entropy method analysis using a simplethree-component grain model shows that seven of the nine sight lines inthe study require a larger fraction of grain materials to be in dustwhen FUV extinction is included in the models. Most of the added grainmaterial is in the form of small (radii <~ 200 Å) grains.Based on observations with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.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 NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| New Runaway O-stars Based on Data from HIPPARCOS
12 new runaway O-stars are identified using an analysis of their propermotions based on data from HIPPARCOS. The peculiar tangential and totaltransverse velocities of these stars are determined. A list of theobserved runaway stars is given.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.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 NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| On the Origin of the High-Ionization Intermediate-Velocity Gas toward HD 14434
We present Far-Ultraviolet Spectroscopic Explorer and Space TelescopeImaging Spectrograph observations of high-ionization interstellarabsorption toward HD 14434 (l=135.1d, b=-3.8d d~2.3 kpc), an O5.5 V starin the Perseus OB1 association. Intermediate-velocity interstellar Si IVand C IV absorption is present at VLSR=-67 km s-1,while low-ionization gas associated with the Perseus arm is detected atabout -50 km s-1. Neither N V nor O VI is detected atVLSR=-67 km s-1, although Al III and Fe III,tracers of warm ionized gas, are seen. The high-ion column densities inthe -67 km s-1 component are log[N(CIV)]=13.92+/-0.02cm-2, log[N(SiIV)]=13.34+/-0.02 cm-2,log[N(NV)]<=12.65 cm-2, and log[N(OVI)]<=13.73cm-2 (3 σ limits). The observed C IV/Si IV ratio of3.8+/-0.3 in this intermediate-velocity cloud (IVC) is similar to theGalactic average (4.3+/-1.9). Our analysis of the Si IV and C IV linewidths yields a temperature of T~10,450+/-3400 K for this component. Atthis low temperature, neither Si IV nor C IV can be produced viacollisions. We investigate several photoionization models to explain theintermediate-velocity Si IV and C IV absorption toward HD 14434.Photoionization models employing cooling of a hot (T~106 K)diffuse plasma as the source of ionizing radiation reproduce theobserved properties of the IVC toward HD 14434 quite well. The hotplasma responsible for the ionizing radiation in these models may beattributed to hot gas contained in a supershell in or near the Perseusarm or from a more generally distributed hot ionized medium.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute.STScI is operated by the Association of Universities for Research inAstronomy, Inc., under the NASA contract NAS 5-26555.
| 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.
| An Atlas of Galactic OB Spectra Observed with the Far Ultraviolet Spectroscopic Explorer
An atlas of far-ultraviolet spectra of 45 Galactic OB stars observedwith the Far Ultraviolet Spectroscopic Explorer is presented. The atlascovers the wavelength region between 912 and 1185 Å with aneffective spectral resolution of 0.12 Å. Systematic trends in themorphology and strength of stellar features are discussed. Particularattention is drawn to the variations of the C III λ1176, S IVλλ1063, 1073, and P V λλ1118, 1128 lineprofiles as a function of temperature and luminosity class; and the lackof a luminosity dependence associated with O VI λλ1032,1038. Numerous interstellar lines are also identified. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer. FUSE is operated for NASA by the Johns Hopkins Universityunder NASA contract NAS5-32985.
| 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.
| Tomographic Separation of Composite Spectra. VII. The Physical Properties of the Massive Triple System HD 135240 (δ Circini)
We present the results of a radial velocity study of the massive,double-lined, O binary HD 135240 based primarily on UV spectroscopy fromthe International Ultraviolet Explorer. Cross-correlation methodsindicate the presence of a third stationary spectral line componentwhich indicates that the system is a triple consisting of a central 3.9day close binary with a distant companion. We measured radial velocitiesfrom the cross-correlation functions after removal of the thirdcomponent, and we combined these with velocities obtained from Hαspectroscopy to reassess the orbital elements. We applied a Dopplertomography algorithm to reconstruct the individual UV spectra of allthree stars, and we determine spectral classifications of O7 III-V, O9.5V, and B0.5 V for the primary, secondary, and tertiary, respectively,using UV criteria defined by Penny, Gies, & Bagnuolo. We comparethese reconstructed spectra to standard single-star spectra to find theUV flux ratios of the components(F2/F1=0.239+/-0.022, andF3/F1=0.179+/-0.021). Hipparcos photometry revealsthat the central pair is an eclipsing binary, and we present the firstmodel fit of the light curve from which we derive an orbitalinclination, i=74deg+/-3deg. This analysisindicates that neither star is currently experiencing Roche lobeoverflow. We place the individual components in the theoretical H-Rdiagram, and we show that the masses derived from the combinedspectroscopic and photometric analysis(Mp/Msolar=21.6+/-2.0 andMs/Msolar=12.4+/-1.0) are significantly lower thanthose computed from evolutionary tracks for single stars.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| 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.
| 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.
| 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.
| Coronal Gas in the Halo. II. ORFEUS Observations of Galactic Halo Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...465..296H&db_key=AST
| Projected Rotational Velocities of O-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...463..737P&db_key=AST
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