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A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way
Without a source of new gas, our Galaxy would exhaust its supply of gasthrough the formation of stars. Ionized gas clouds observed at highvelocity may be a reservoir of such gas, but their distances are key forplacing them in the galactic halo and unraveling their role. We haveused the Hubble Space Telescope to blindly search for ionizedhigh-velocity clouds (iHVCs) in the foreground of galactic stars. Weshow that iHVCs with 90 ? |vLSR| ? 170 kilometers persecond (where vLSR is the velocity in the local standard ofrest frame) are within one galactic radius of the Sun and have enoughmass to maintain star formation, whereas iHVCs with |vLSR|? 170 kilometers per second are at larger distances. These may bethe next wave of infalling material.
| Ejection velocities of high Galactic latitude runaway stars
We estimate the distribution of ejection velocities for the knownpopulation of high Galactic latitude runaway stars. The initial sampleis a collection of 174 early-type stars selected from the literature.The stars are first classified according to their evolutionary status inorder to obtain a homogeneous sample of 96 genuine main-sequence stars.Their present velocities and flight times are then estimated usingproper motion data from various astrometric catalogues (includingTycho-2, UCAC2 and USNO-B) and the ejection velocities are computed bytracing their orbits back in time, based on a Galactic potential. Thepotential used is constructed from a mass density model chosen to fitthe most recent observational constraints.We find evidence for two different populations of runaway stars: a'high' velocity population, with a maximum ejection velocity of about400-500 km s-1, and a 'low' velocity population, with amaximum ejection velocity of about 300 km s-1. We argue thatthe observed limit of 500 km s-1 and the bimodality of theobserved ejection velocity distribution are natural consequences of theso-called Binary Ejection Mechanism. We discuss the connection betweenthe 'high' velocity population and the so-called hypervelocity stars,showing how previously studied hypervelocity stars are consistent withthe results obtained.We also find that some stars that were once thought to be best explainedas being formed in the halo are compatible with a runaway hypothesisonce proper motions are included in the analysis. However, three starsin the selected sample appear to be inconsistent with ejection from theGalactic disc. Possible scenarios are discussed, including a possibleformation in the Galactic halo.
| A High-metallicity, High-velocity Cloud Along the Mrk 421 Sight Line: A Tracer of Complex M?
We present a new measurement, 0.85-3.5 Z sun, of themetallicity of high-velocity cloud (HVC) Complex M by analyzingultraviolet spectroscopic observations of the blazar Mrk 421 taken withthe Cosmic Origins Spectrograph on the Hubble Space Telescope and theFar-Ultraviolet Spectroscopic Explorer. Although an HVC at VLSR = -131 km s-1 is not visible in 21 cm emission(log N H I < 18.38; 3?), it is detected inultraviolet absorption lines of C II, N I, O I, O VI, Si II, Si III, SiIV, Fe II, and H I. By referencing velocities to theintermediate-velocity cloud at -60 km s-1 and jointlyanalyzing H I absorption from high-order H I Lyman lines, we measure logN H I = 16.84+0.34 -0.13 (1?) inthe HVC. Comparing H I and O I, we find an HVC metallicity [O/H]=0.32+0.22 -0.39. Because the sight line passes~4° from the HVCs in Complex M, the detected HVC may represent thehighest velocity component of the Complex, and our measurements providea lower limit to its metallicity. The high, possibly super-solarmetallicity, together with the low distance, z < 3.5 kpc, above theGalactic plane suggests that Complex M is condensed returning gas from aGalactic fountain.
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| 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.
| Runaway and Hypervelocity Stars in the Galactic Halo: Binary Rejuvenation and Triple Disruption
Young stars observed in the distant Galactic halo are usually thought tohave formed elsewhere, either in the Galactic disk or perhaps theGalactic center (GC), and subsequently ejected at high velocities totheir current position. However, some of these stars have apparentlifetimes shorter than the required flight time from the Galacticdisk/GC. We suggest that such stars have evolved in close runaway orhypervelocity binaries. Stellar evolution of such binaries can drivethem into mass transfer configurations and even mergers. Such evolutioncould then rejuvenate them (e.g., blue stragglers) and extend theirlifetime after their ejection. The extended lifetimes of such starscould then be reconciled with their flight times to the Galactic halo.We study the possibilities of binary runaway and hypervelocity stars(HVSs) and show that such binaries could have been ejected in tripledisruptions and other dynamical interactions with stars or with massiveblack holes (MBHs). We show that currently observed "too young" star inthe halo could have been ejected from the Galactic disk or the GC and beobservable in their current position if they were ejected as binaries.Specifically, it is shown that the HVS HE 0437-5439 could besuch a rejuvenated star. Other suggestions for its ejection from theLarge Magellanic Cloud are found to be highly unlikely. Moreover, it isshown that its observed metallicity is most consistent with a Galacticorigin and a GC origin cannot currently be ruled out. In addition, wesuggest that triple disruptions by the MBH in the GC could also capturebinaries in close orbits near the MBH, some of which may later evolve tobecome more massive rejuvenated stars.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way
To probe the distribution and physical characteristics of interstellargas at temperatures T~3×105 K in the disk of the MilkyWay, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) toobserve absorption lines of O VI λ1032 toward 148 early-typestars situated at distances >1 kpc. After subtracting off a mildexcess of O VI arising from the Local Bubble, combining our new resultswith earlier surveys of O VI, and eliminating stars that showconspicuous localized X-ray emission, we find an average O VI midplanedensity n0=1.3×10-8 cm-3. Thedensity decreases away from the plane of the Galaxy in a way that isconsistent with an exponential scale height of 3.2 kpc at negativelatitudes or 4.6 kpc at positive latitudes. Average volume densities ofO VI along different sight lines exhibit a dispersion of about 0.26 dex,irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size anddensity, but instead is distributed in regions that have a very broadrange of column densities, with the more strongly absorbing cloudshaving a lower space density. Line widths and centroid velocities aremuch larger than those expected from differential Galactic rotation, butthey are nevertheless correlated with distance and N(O VI), whichreinforces our picture of a diverse population of hot plasma regionsthat are ubiquitous over the entire Galactic disk. The velocity extremesof the O VI profiles show a loose correlation with those of very stronglines of less ionized species, supporting a picture of a turbulent,multiphase medium churned by shock-heated gas from multiple supernovaexplosions.
| The Nonisothermality and Extent of Galactic Diffuse Hot Gas toward Markarian 421
Diffuse hot gas can be traced effectively by its X-ray absorption andemission. We present a joint analysis of these tracers to characterizethe spatial and temperature distributions of the Galactic hot gas alongthe sight line toward the nearby bright active galactic nucleus Mrk 421.We also complement this analysis with far-UV O VI absorptionobservations. We find that the observed absorption line strengths of OVII and O VIII are inconsistent with the diffuse backgroundemission-line ratio of the same ions, if the gas is assumed to beisothermal in a collisional ionization equilibrium state. But all theselines as well as the diffuse 3/4 keV broadband background intensity inthe field can be fitted with a plasma with a power-law temperaturedistribution. We show that this distribution can be derived from a hotgaseous disk model with the gas temperature and density decreasingexponentially with the vertical distance from the Galactic plane. Thejoint fit gives the exponential scale heights as ~1.0 and 1.6 kpc andthe middle plane values as 2.8×106 K and2.4×10-3 cm-3 for the temperature anddensity, respectively. These values are consistent with those inferredfrom X-ray observations of nearby edge-on galaxies similar to our own.
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. II. Kinematics and Full Sample Analysis
This paper continues the analysis of faint high-latitude B stars fromMartin. Here we analyze the kinematics of the stars and combine themwith the abundance information from the first paper to classify eachone. The sample contains 31 Population I runaways, 15 old evolved stars(including 5 blue horizontal-branch [BHB] stars, 3 post-HB stars, 1pulsating helium dwarf, and 6 stars of ambiguous classification), 1 Fdwarf, and 2 stars that do not easily fit in one of the othercategories. No star in the sample unambiguously shows thecharacteristics of a young massive star formed in situ in the halo. Thetwo unclassified stars are probably extreme Population I runaways. Thelow binary frequency and rotational velocity distribution of thePopulation I runaways imply that most were ejected from dense starclusters by the dynamic ejection scenario. However, we remain puzzled bythe lack of runaway Be stars. We also confirm that PB 166 and HIP 41979are both 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.
| A Catalog of Field Horizontal Branch Stars Aligned with High-Velocity Clouds
We present a catalog of 430 field horizontal branch (FHB) stars,selected from the Hamburg/ESO Survey (HES), which fortuitously alignwith high column density neutral hydrogen (H I) high-velocity cloud(HVC) gas. These stars are ideal candidates for absorption-line studiesof HVCs, attempts at which have been made for almost 40 years withlittle success. A parent sample of 8321 HES FHB stars was used toextract H I spectra along each line of sight, using the H I ParkesAll-Sky Survey. All lines of sight aligned with high-velocity H Iemission with peak brightness temperatures greater than 120 mK wereexamined. The H I spectra of these 430 probes were visually screened andcross-referenced with several HVC catalogs. In a forthcoming paper, wereport on the results of high-resolution spectroscopic observations of asample of stars drawn from this catalog.
| FUSE Observations of Interstellar and Intergalactic Absorption toward the X-Ray-bright BL Lacertae Object Markarian 421
High-quality Far Ultraviolet Spectroscopic Explorer (FUSE) observationsat 20 km s-1 resolution of interstellar and intergalacticabsorption from 910 to 1187 Å are presented for the X-ray-brightBL Lac object Mrk 421. These observations are supplemented with FUSEdata for the distant halo stars BD +38°2182 and HD 93521 near theMrk 421 line of sight, in order to obtain information about the distanceto absorbing structures in the Milky Way toward Mrk 421. The FUSE ISMobservations provide measures of absorption by O VI and many otherspecies commonly found in warm neutral and warm ionized gas, including HI, C II, C III, O I, N I, N II, Fe II, and Fe III. In this study weconsider the O VI absorption between -140 and 165 km s-1 andits relationship to the lower ionization absorption and strongabsorption produced by O VII and O VIII at X-ray wavelengths. The O VIabsorption extending from -140 to 60 km s-1 is associatedwith strong low-ionization gas absorption and originates in the Galacticthick disk/halo. This O VI appears to be produced by a combination ofprocesses, including conductive interfaces between warm and hot gas andpossibly cooling Galactic fountain gas and hot halo gas bubbles. The OVI absorption extending from 60 to 165 km s-1 has unusualionization properties in that there is very little associatedlow-ionization absorption, with the exception of C III. This absorptionis not observed toward the two halo stars, implying that it occurs ingas more distant than 3.5 kpc from the Galactic disk. Over the 60-165 kms-1 velocity range, O VI and C III absorption have the samekinematic behavior. The ratio N(OVI)/N(CIII)=10+/-3 over the 60-120 kms-1 velocity range. Given the association of O VI with C III,it is unlikely that the high-velocity O VI coexists with the hotter gasresponsible for the O VII and O VIII absorption. The O VI positivevelocity absorption wing might be tracing cooler gas entrained in a hotGalactic fountain outflow. The O VII and O VIII absorption observed byChandra and XMM-Newton may trace the hot gas in a highly extended (~100kpc) Galactic corona or hot gas in the Local Group. The low resolutionof the current X-ray observations (~750-900 km s-1) and thekinematical complexity of the O VI absorption along typical lines ofsight through the Milky Way halo make it difficult to clearly associatethe O VI absorption with that produced by O VII and O VIII. A search formetal lines associated with the Lyα absorber at z=0.01, which issituated in a galactic void, was unsuccessful.
| 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.
| Synthetic High-Resolution Line Spectra of Star-forming Galaxies below 1200 Å
We have generated a set of far-ultraviolet stellar libraries usingspectra of OB and Wolf-Rayet stars in the Galaxy and the Large and SmallMagellanic Cloud. The spectra were collected with the Far UltravioletSpectroscopic Explorer and cover a wavelength range from 1003.1 to1182.7 Å at a resolution of 0.127 Å. The libraries extendfrom the earliest O to late-O and early-B stars for the Magellanic Cloudand Galactic libraries, respectively. Attention is paid to the complexblending of stellar and interstellar lines, which can be significant,especially in models using Galactic stars. The most severe contaminationis due to molecular hydrogen. Using a simple model for the H2line strength, we were able to remove the molecular hydrogen lines in asubset of Magellanic Cloud stars. Variations of the photospheric andwind features of C III λ1176, O VI λλ1032, 1038, PV λλ1118, 1128, and S IV λλ1063, 1073, 1074are discussed as a function of temperature and luminosity class. Thespectral libraries were implemented into the LavalSB and Starburst99packages and used to compute a standard set of synthetic spectra ofstar-forming galaxies. Representative spectra are presented for variousinitial mass functions and star formation histories. The valid parameterspace is confined to the youngest ages of less than ~=10 Myr for aninstantaneous burst, prior to the age when incompleteness of spectraltypes in the libraries sets in. For a continuous burst at solarmetallicity, the parameter space is not limited. The suite of models isuseful for interpreting the rest-frame far-ultraviolet in local andhigh-redshift galaxies. Based on observations made with theNASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operatedfor NASA by the Johns Hopkins University under NASA contract NAS5-32985.
| 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.
| High-Velocity Clouds as a Probe of the Global Structure of the Gaseous Galactic Halo
We construct a simplified pressure equilibrium model for the confinementof high-velocity clouds (HVCs) in the Galactic halo of the Milky Way.The ambient pressure is obtained from a model for the distribution ofthe coronal gas density. This is assumed as an isothermal plasma inhydrostatic equilibrium with the gravitational field of the Galaxy. Thecloud internal pressure, for either subsonic or supersonic HVCs, isexpressed in terms of its observed parameters and as a function ofdistance from the Sun. The distances to three HVCs in complex M,observationally determined by Danly and coworkers in 1993, are used toset the values of the two free parameters of the model, namely, thethermal pressure of the coronal gas in the solar vicinity(~4×10-13 dyn cm-2) and its temperatureTg~106 K. This model can be more rigorously testedwhen a set of cloud distances, velocities, velocity dispersion, columndensities, and angular sizes becomes available.
| 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.
| H I Spectra and Column Densities toward HVC and IVC Probes
We show 21 cm line profiles in the direction of stars and extragalacticobjects, lying projected on high- and intermediate-velocity clouds (HVCsand IVCs). About half of these are from new data obtained with theEffelsberg 100 m telescope, about a quarter are extracted from theLeiden-Dwingeloo Survey (LDS), and the remaining quarter were observedwith other single-dish telescopes. H I column densities were determinedfor each HVC/IVC. Paper I of this series uses these in combination withoptical and ultraviolet high-resolution measurements to deriveabundances. Here an analysis is given of the difference and ratio of N(HI) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratioN(H I-9')/N(H I-35') lies in the range 0.2-2.5. For low-velocity gasthis ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, butit appears that the correction for stray radiation is slightly off). Thesmaller range for the low-velocity gas may be caused by confusion in theline of sight, so that a low ratio in one component can be compensatedby a high ratio in another-for 11 low-velocity clouds fitted by onecomponent the distribution of ratios has a larger dispersion. Comparisonwith higher angular resolution data is possible for 16 sight lines.Eight sight lines with H I data at 1'-2' resolution show a range of0.75-1.25 for N(H I-2')/N(H I-9'), while in eight other sight lines N(HI-Lyα)/N(H I-9') ranges from 0.74 to 0.98.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| Star-forming Processes Far from the Galactic Disk: Inoperative or Indolent Where Operative
Highly supersonic collisions between gaseous clouds may effectivelytrigger star formation in the disk of our Galaxy, but not in the diffuseenvironment of the Galactic halo. This is because the observedhigh-velocity clouds (HVCs) are not dominated by collisions: thecharacteristic time between cloudlet collisions inside an HVC at anassumed distance of 10 kpc is at least 1 Gyr for collective encountersand at least 10 Gyr if a particular cloudlet is considered. In agreementwith this result, we also estimate that the observed cloudlets containsmaller masses than the nonmagnetic Jeans mass that signals favorableconditions for gravitational collapse and further fragmentation in theisothermal regime. The diffuse environment observed around theMagellanic Clouds (MCs) is more difficult to understand than HVCs. Sixsparse blue associations and two young B-type stars have been observedin the H I bridge between the MCs, while no stars exist in the H I cloudcomplexes that make up the Magellanic Stream. We discuss the conditionsunder which spatially sporadic star formation took place in theMagellanic Bridge during the past 16--25 Myr and the reasons for thecomplete absence of star formation in the Stream during its entirelifetime. We also estimate the angular resolutions that need to beachieved by follow-up radio observations of these regions that coulddetect cold cloudlets embedded in the gas.
| The distance to Complex M and the Intermediate Velocity Arch
The Intermediate Velocity Arch is an association of Hi clouds lyingwithin the broad limits l~110 deg-210 deg,b~50 deg-80 deg. We usehigh-resolution optical, and single-dish 21-cm, observations of 10early-type halo star sightlines in this region to establish distancelimits (0.4<=z<=3.5kpc) for seven IVCs that are part of the Arch.We re-examine a previous determination of upper and lower distancelimits on the IVC Complex M using the closely aligned halo starsBD+382182 and HD93521. Our Hi spectra show no evidence of emission fromComplex M, leading us to conclude that, while an upper distance limit ofz<=3.5 kpc may be established, no useful lower distance determinationcan be made. Our new Hi observations allow new limits to be establishedon the metallicity of Complex M, and show C, O and Si variations withrespect to solar abundances in the range +/-0.3 dex.
| High-Velocity Rain: The Terminal Velocity Model of Galactic Infall
A model is proposed for determining the distances to fallinginterstellar clouds in the galactic halo by measuring the cloud velocityand column density and assuming a model for the vertical densitydistribution of the Galactic interstellar medium. It is shown thatfalling clouds with N(H I) <~ 1019 cm-2 may be decelerated to aterminal velocity which increases with increasing height above theGalactic plane. This terminal velocity model correctly predicts thedistance to high-velocity cloud Complex M and several other interstellarstructures of previously determined distance. It is demonstrated howinterstellar absorption spectra alone may be used to predict thedistances of the clouds producing the absorption. If the distance,velocities, and column densities of enough interstellar clouds are knownindependently, the procedure can be reversed, and the terminal velocitymodel can be used to estimate the vertical density structure (both themean density and the porosity) of the interstellar medium. Using thedata of Danly and assuming a drag coefficient of CD ≅ 1, thederived density distribution is consistent with the expected densitydistribution of the warm ionized medium, characterized by Reynolds.There is also evidence that for z >~ 0.4 kpc one or more of thefollowing occurs: (1) the neutral fraction of the cloud decreases to ~31+/- 14%, (2) the density drops off faster than characterized byReynolds, or (3) there is a systematic decrease in CD with increasing z.Current data do not place strong constraints on the porosity of theinterstellar medium.
| High-Velocity Clouds
High-velocity clouds (HVCs) consist of neutral hydrogen (\HI) atvelocities incompatible with a simple model of differential galacticrotation; in practice one uses {the absolue value of} VLSR {greater orequal to} 90 km/s to define HVCs. This review describes the mainfeatures of the sky and velocity distributions, as well as the availableinformation on cloud properties, small-scale structure, velocitystructure, and observations other than in 21-cm emission. We show thatHVCs contain heavy elements and that the more prominent ones are morethan 2 kpc from the Galactic plane. We evaluate the hypotheses proposedfor their origin and reject those that account for only one or a fewHVCs. At least three different hypotheses are needed: one for theMagellanic Stream and possibly related clouds, one for the Outer ArmExtension, and one (or more) for the other HVCs. We discuss the evidencefor the accretion and the fountain model but cannot rule out either one.
| The Distance to Two Neutral Hydrogen Clouds: The High-Velocity Complex A and the Low-Latitude Intermediate-Velocity Cloud
A lower limit to the distance of the high-velocity cloud (HVC) complex Aof 4 kpc (z > 3 kpc) is derived. The HVC is detected toward theSeyfert galaxy Mrk 106 in Mg II ??2796, 2803 absorptionspectra taken with Hubble Space Telescope (HST) proving that Mg^+^ ispresent in the cloud. It is not detected in the Mg II spectra of twostars, PG 0859+593 (distance 4 kpc) and PG 0906+597 (distance 0.7 kpc).The distances to the stars are derived by matching Stromgren photometryand intermediate-resolution spectroscopy with model stellar atmospheres;they are estimated to be accurate to within 1 kpc. From a combination ofEffelsberg data and Westerbork maps with 2' or 3' resolution, we showthat the H I column density and thus the Mg^+^ abundance in thedirection of the two stars is sufficiently high for the nondetections toimply that the HVC is behind the stars. This distance limit can be usedto eliminate several recent models for complex A that placed it nearby.We also derive a distance bracket of 1.7 < d < 4 kpc (1.1 < z< 3 kpc) for an intermediate-velocity cloud (IVC) at velocities ofabout -50 km s^-1^. This IVC was named the Low-LatitudeIntermediate-Velocity Arch by Kuntz & Danly and is seen between l =120^deg^-160^deg^ and b = 30^deg^- 45^deg^.
| GHRS Observations of the ISM Toward BD +38 2182
GHRS Echelle and G160M data toward the halo star BD +38 2182 arepresented. The data reveal several distinct components, often withdramatically different column density ratios in the diagnostic UV ions.The data are evaluated in terms of thermal and photo-ionization models.
| Intermediate-Velocity Gas in the North Galactic Hemisphere: H i Studies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..703K&db_key=AST
| The Ca+ abundance of HVC complex C
We report the first detection of Ca II H and K absorption in thehigh-velocity cloud complex C, the largest HVC. Absorption is detectedin the spectra of the quasar PG 1351+640 and of the Seyfert galaxy Mark290, at velocities of -163 and -137km/s (see fig.2). The implied Ca+abundances are about 2x10^-8^, or about 0.01 times the total solarCalcium abundance. This values lies near the middle of the range ofvalues found for other HVCs. The measured abundance will allow firmconclusions about the distance of complex C from the presence or absenceof its absoprtion in stellar spectra.
| 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.
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Observation and Astrometry data
| Constellation: | Leo Minor |
| Right ascension: | 10h49m12.91s |
| Declination: | +38°00'14.8" |
| Apparent magnitude: | 11.134 |
| Proper motion RA: | -11.1 |
| Proper motion Dec: | -1.8 |
| B-T magnitude: | 10.912 |
| V-T magnitude: | 11.116 |
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