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The Multiphase Absorption Systems toward PG 1206+459
A high-resolution (R=30,000) ultraviolet spectrum is presented, whichcovers Lyα and many low-, intermediate-, and high-ionizationtransitions in the three Mg II-selected absorption systems toward thequasar PG 1206+459. Three systems (A, B, and C), which are clusteredwithin 1500 km s-1 at z~0.93, were originally identified in aspectrum obtained with the High Resolution Spectrograph (HIRES) on theKeck I Telescope. A WIYN (Wisconsin-Indiana-Yale-NOAO) Gunn i-band imageof the quasar field and spectroscopy of two galaxy candidates arepresented. A multiphase medium is seen in all three systems, consistentwith smaller, denser clouds producing low-ionization transitions (Mg II,Fe II, and Si II) and larger, diffuse, photoionized clouds giving riseto higher ionization transitions (C IV, N V, and/or O VI). (1) System A,a multicloud, weak Mg II absorber at z=0.9254, requires a supersolarmetallicity in both low- and high-ionization phases, unless anα-group enhancement is included. The low-ionization absorption isproduced in clouds with sizes of 10-70 pc, which are surrounded invelocity space by broader, high-ionization components. With theunusually complex velocity structure resolved in the N V profiles, thissystem is unlikely to represent a traditional galaxy disk/corona. Themost likely candidate host galaxy is a ~2L*, apparently warped, spiralat an impact parameter of 43 h-1 kpc. (2) System B, atz=0.9276, has the strongest Mg II absorption and has an approximatelysolar metallicity in the low-ionization phase. The smooth, broadhigh-ionization profiles may indicate a coronal structure similar tothat of the Milky Way. The redshift of an L* galaxy (z=0.9289), at animpact parameter of 38 h-1 kpc, is consistent with theredshift of this system. (3) System C, at z=0.9342, has a singlecomponent in Mg II, separated from the other two systems by ~+1000 kms-1. The Lyα profile is not aligned with the Mg II,requiring an additional velocity component offset by -40 kms-1. System C lacks the small, low-ionization cloudcharacteristic of an isolated single-cloud, weak Mg II absorber. Itsabsorption properties are similar to the ``satellite clouds'' of classicstrong Mg II absorbers, so this could be a high-velocity cloud in thegalaxy group responsible for the systems, possibly related to a 0.2L*galaxy at an impact parameter of 43 h-1 kpc.Based in part on observations obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among Caltech, theUniversity of California, and NASA. The Observatory was made possible bythe generous financial support of the W. M. Keck Foundation.
| 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.
| 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 study of the behaviour of the NaI/KI column density ratio in the interstellar medium using the Na ultraviolet doublet
Here we make a new study of the behaviour of the NaI/KI column densityratio in the interstellar medium, using a sample of new observations of28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and1997, and previously published observations (obtained by some of theauthors) of 21 stars. The sightlines cover a range of distances anddirections, including into the Galactic halo. We make use of newobservations of the NaI ultraviolet (UV) doublet for some 18 stars. Thisdoublet is much weaker than the NaI D doublet and so is less susceptibleto saturation effects, and it is well known that it can be used toobtain more accurate NaI column densities with a smaller error range. Wefind an average N(NaI)/N(KI) ratio from the NaI UV data of about 90,which is rather higher than that found previously by Hobbs and Lequeux.The Na UV-KI ratio shows a small increase in value with increasingcolumn density, while we also find a sample of low N(NaI)/N(KI) ratioclouds generally seen towards distant objects on high-latitudesightlines that reach into the halo, so that the ratio decreases moresharply at lower column densities. As the values of the ratio for thesehalo clouds (10-20) bracket the cosmic Na/K abundance ratio, we suggestthat these ratios result from a harder radiation field in the lowerhalo, such that the ionized fractions of NaI and KI become similar.Clearly caution needs to be applied in using any kind of `standardvalue' for the NaI/KI column density ratio.
| Wind variability of B supergiants. IV. A survey of IUE time-series data of 11 B0 to B3 stars
We present the most suitable data sets available in the InternationalUltraviolet Explorer (IUE) archive for the study of time-dependentstellar winds in early B supergiants. The UV line profile variability in11 B0 to B3 stars is analysed, compared and discussed, based on 16separate data sets comprising over 600 homogeneously reducedhigh-resolution spectrograms. The targets include ``normal'' stars withmoderate rotation rates and examples of rapid rotators. A gallery ofgrey-scale images (dynamic spectra) is presented, which demonstrates therichness and range of wind variability and highlights differentstructures in the winds of these stars. This work emphasises thesuitability of B supergiants for wind studies, under-pinned by the factthat they exhibit unsaturated wind lines for a wide range of ionization.The wind activity of B supergiants is substantial and has highly variedcharacteristics. The variability evident in individual stars isclassified and described in terms of discrete absorption components,spontaneous absorption, bowed structures, recurrence, and ionizationvariability and stratification. Similar structures can occur in stars ofdifferent fundamental parameters, but also different structures mayoccur in the same star at a given epoch. We discuss the physicalphenomena that may be associated with the spectral signatures. Thediversity of wind patterns evident likely reflects the role of stellarrotation and viewing angle in determining the observationalcharacteristics of azimuthally extended structure rooted at the stellarsurface. In addition, SEI line-synthesis modelling of the UV wind linesis used to provide further information about the state of the winds inour program stars. Typically the range, implied by the line profilevariability, in the product of mass-loss rate and ion fraction (mdotq_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ;it can however be several times larger over localised velocity regions.At a given effective temperature the mean relative ion ratios can differby a factor of 5. The general excess in predicted (forward-scattered)emission in the low velocity regime is discussed in terms of structuredoutflows. Mean ion fractions are estimated over the B0 to B1 spectralclasses, and trends in the ionic ratios as a function of wind velocityare described. The low values obtained for the ion fractions of UVresonance lines may reflect the role of clumping in the wind.
| 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.
| 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.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| B Stars as a Diagnostic of Star Formation at Low and High Redshift
We have extended the evolutionary synthesis models by Leitherer et al.by including a new library of B stars generated from the IUEhigh-dispersion spectra archive. We present the library and show how thestellar spectral properties vary according to luminosity classes andspectral types. We have generated synthetic UV spectra for prototypicalyoung stellar populations varying the IMF and the star formation law.Clear signs of age effects are seen in all models. The contribution of Bstars in the UV line spectrum is clearly detected, in particular forgreater ages when O stars have evolved. With the addition of the newlibrary we are able to investigate the fraction of stellar andinterstellar contributions and the variation in the spectral shapes ofintense lines. We have used our models to date the spectrum of the localsuper-star cluster NGC 1705-1. Photospheric lines of C III λ1247,Si III λ1417, and S V λ1502 were used as diagnostics todate the burst of NGC 1705-1 at 10 Myr. Interstellar lines are clearlyseen in the NGC 1705-1 spectrum. Broadening and blueshifts of severalresonance lines are stronger in the galaxy spectrum than in our modelsand are confirmed to be intrinsic of the galaxy. Si II λ1261 andAl II λ1671 were found to be pure interstellar lines with anaverage blueshift of 78 km s-1 owing to a directed outflow ofthe interstellar medium. We have selected the star-forming galaxy1512-cB58 as a first application of the new models to high-z galaxies.This galaxy is at z=2.723, it is gravitationally lensed, and its highsignal-to-noise ratio Keck spectrum shows features typical of localstarburst galaxies, such as NGC 1705-1. Models with continuous starformation were found to be more adequate for 1512-cB58 since there arespectral features typical of a composite stellar population of O and Bstars. A model with Z=0.4 Zsolar and an IMF with α=2.8reproduces the stellar features of the 1512-cB58 spectrum.
| 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.
| GHRS and ORFEUS II Observations of the Highly Ionized Interstellar Medium toward ESO 141-055
We present Goddard High Resolution Spectrograph (GHRS) and ORFEUS IImeasurements of Si IV, C IV, N V, and O VI absorption in theinterstellar medium of the Galactic disk and halo toward the nucleus ofthe Seyfert galaxy ESO 141-055. The high-ionization absorption isstrong, with line strengths consistent with the spectral signatureexpected for hot (T>~10^5 K) collisionally ionized gas in either a``Galactic fountain'' or an inhomogeneous medium containing a mixture ofconductive interfaces and turbulent mixing layers. The total O VI columndensity of ~10^15 cm^-2 suggests that the scale height of O VI is large(>~3 kpc) in this direction. Models of the C IV velocity distributionalong the sight line are consistent with a large scale height for thehighly ionized gas. Comparison of the high ion column densities withmeasurements for other sight lines indicates that the highly ionized gasdistribution is patchy. The amount of O VI perpendicular to the Galacticplane varies by at least a factor of ~4 among the complete halo sightlines thus far studied. In addition to the high ion absorption, lines oflow-ionization species are also present in the spectra. With thepossible exception of Ar I, which may have a lower than expectedabundance resulting from partial photoionization of gas along the sightline, the absorption strengths are typical of those expected for thewarm, neutral interstellar medium. The sight line intercepts a coldmolecular cloud with N(H_2)~10^19 cm^-2. The cloud has an identifiablecounterpart in IRAS 100 μm emission maps of this region of the sky.We detect a Lyα absorber associated with ESO 141-055 at z=0.03492.This study presents an enticing glimpse into the interstellar andintergalactic absorption patterns that will be observed at high spectralresolution by the Far Ultraviolet Spectroscopic Explorer.
| 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.
| Simulations of Supernova Remnants in Diffuse Media and Their Application to the Lower Halo of the Milky Way. I. The High-Stage Ions
This paper reports on detailed, nonequilibrium hydrodynamic simulationsof supernova remnants (SNRs) evolving in a warm, low-density, nonthermalpressure-dominated ambient medium (T = 104 K, n = 0.01 cm-3, Pnt = 1.8 x103 K cm-3), with the goals of characterizing their structure and C+3,N+4, and O+5 content, emission, and line profiles and investigating theeffects of supernova remnants in the lower Galactic halo. If undisturbedby external objects, these remnants have great longevity, surviving for~1.7 x 107 yr. During the adiabatic phase, they contain large quantitiesof C+3, N+4, and O+5 in the hot gas behind their shock fronts. They emitbrightly in the ultraviolet resonance lines and would appear edgebrightened to observations of column density and emission. At the end ofthe adiabatic phase, each SNR develops a zone of cooling and recombiningC+3, N+4, and O+5 in the transition region between the hot bubble andthe cool shell. The resonance line luminosities plummet, and the edgebrightening diminishes. As the remnants evolve, the interiors coolfaster than the ions can recombine to their equilibrium levels. Thus,during most of the remnants' lifetimes the C+3 line widths are smallerthan expected from collisional equilibrium, and after the remnants havecompletely cooled, some C+3 remains. The O+5, N+4, and C+3 distributionsoverlap incompletely. The O+5 ions are more plentiful in the warmer gasat smaller radii than are the N+4 or C+3 ions. As a result, after theshell forms the thermal pressure in the O+5-rich gas is at least twiceas large as that in the C+3-rich gas. During most of its lifetime, theremnant's interior is less than 106 K. Therefore, the fraction of areacovered or volume filled by very hot SNR gas is much smaller than thatfilled by warm SNR gas. These simulations have been combined with thestatistical distribution of isolated supernova progenitors in order toderive rough estimates of the appearance of the ensemble of isolatedsupernova remnants in the lower halo. The agreement between thesimulation results and observational results in terms of average columndensity and spatial patchiness shows that much, if not all, of thehigh-latitude O+5, N+4, and C+3 between the local bubble and roughly akiloparsec can be attributed to isolated SNRs in the lower halo. Thesimulations may also be of interest to studies of the external galaxiesand the hypothesis that the Local Bubble is a single supernova remnantevolving in a low-density ambient medium.
| The structure of the interstellar gas towards stars in the globular cluster NGC 6541
Observations are presented of high resolution interstellar Na I D and KI (lambda 7699 Angstroms) line profiles towards 7 stars in the globularcluster NGC 6541 and 3 field stars within 3°\ ofthe cluster. It is seen that the gas in this general direction (l, b ~350°, -11°) shows a complex structure, having the greatestnumber of components yet identified in one of our cluster sightlines,with absorption components detected over an LSR velocity range ~ -100 to+100 km s(-1) . We consider, as far as possible, the identification ofthe observed gas components. In the low-velocity gas we identify theextensive and nearby diffuse gas which contains the cold, dark andmolecular clouds observed closer to the Galactic plane. Components aretentatively identified with both positive and negative peculiar velocitygas components produced by outlying gas in the Sagittarius arm, with gasin the Scutum-Crux and the 3 kpc arms, with an H I filament detected inradio mapping and with shell-like structures expanding from the Sco-Cenassociation. From the cluster star spectra some variability in the finescale gas structure is seen on a scale of a few arcsecs.
| 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.
| 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.
| 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 S/N Echelle spectroscopy in young stellar groups. II. Rotational velocities of early-type stars in SCO OB2.
We investigate the rotational velocities of early-type stars in the ScoOB2 association. We measure v.sin(i) for 156 established and probablemembers of the association. The measurements are performed with threedifferent techniques, which are in increasing order of expectedv.sin(i): 1) converting the widths of spectral lines directly tov.sin(i), 2) comparing artificially broadened spectra of low v.sin(i)stars to the target spectrum, 3) comparing the HeI λ4026 lineprofile to theoretical models. The sample is extended with literaturedata for 47 established members of Sco OB2. Analysis of the v.sin(i)distributions shows that there are no significant differences betweenthe subgroups of Sco OB2. We find that members of the binary populationof Sco OB2 on the whole rotate more slowly than the single stars. Inaddition, we find that the B7-B9 single star members rotatesignificantly faster than their B0-B6 counterparts. We test varioushypotheses for the distribution of v.sin(i) in the association. Theresults show that we cannot clearly exclude any form of randomdistribution of the direction and/or magnitude of the intrinsicrotational velocity vector. We also investigate the effects of rotationon colours in the Walraven photometric system. We show that positions ofB7-B9 single dwarfs above the main sequence are a consequence ofrotation. This establishes the influence of rotation on the Walravencolours, due primarily to surface gravity effects.
| 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.
| 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
| Hubble Space Telescope Observations of Interstellar Lines in Three High-Latitude Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...462..758J&db_key=AST
| 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
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Terminal Velocities and the Bistability of Stellar Winds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...455..269L&db_key=AST
| 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
| Discovery of Highly Ionized High-Velocity Clouds toward Markarian 509
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...451..616S&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
| Metal Abundances and Physical Conditions in Two Damped LY alpha Systems toward HS 1946+7658
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...447..597L&db_key=AST
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Observation and Astrometry data
| Constellation: | Στέφανος Νότιος |
| Right ascension: | 18h18m40.00s |
| Declination: | -42°17'18.0" |
| Apparent magnitude: | 6.3 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | 3.2 |
| Proper motion Dec: | -4.4 |
| B-T magnitude: | 6.106 |
| V-T magnitude: | 6.266 |
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