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Long term photometric monitoring with the Mercator telescope. Frequencies and mode identification of variable O-B stars
Aims. We selected a large sample of O-B stars that were considered as(candidate) slowly pulsating B, β Cep, and Maia stars after theanalysis of their hipparcos data. We analysed our new seven passbandgeneva data collected for these stars during the first three years ofscientific operations of the mercator telescope. We performed afrequency analysis for 28 targets with more than 50 high-qualitymeasurements to improve their variability classification. For thepulsating stars, we tried both to identify the modes and to search forrotationally split modes. Methods: . We searched for frequenciesin all the geneva passbands and colours by using two independentfrequency analysis methods and we applied a 3.6 S/N-level criterion tolocate the significant peaks in the periodograms. The modes wereidentified by applying the method of photometric amplitudes for which wecalculated a large, homogeneous grid of equilibrium models to perform apulsational stability analysis. When both the radius and the projectedrotational velocity of an object are known, we determined a lower limitfor the rotation frequency to estimate the expected frequency spacingsin rotationally split pulsation modes. Results: . We detected 61frequencies, among which 33 are new. We classified 21 objects aspulsating variables (7 new confirmed pulsating stars, including 2 hybridβ Cep/SPB stars), 6 as non-pulsating variables (binaries or spottedstars), and 1 as photometrically constant. All the Maia candidates werereclassified into other variability classes. We performed modeidentification for the pulsating variables for the first time. The mostprobable ℓ value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes,respectively, including only 4 unambiguous identifications. For 7 starswe cannot rule out that some of the observed frequencies belong to thesame rotationally split mode. For 4 targets we may begin to resolveclose frequency multiplets. Based on observations collected with the p7 photometer attached to theFlemish 1.2-m mercator telescope situated at the Roque de los Muchachosobservatory on La Palma (Spain). Section [see full textsee full text],including Figs. is only available in electronic form athttp://www.aanda.org, and Tables 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| 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.
| 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.
| 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.
| 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.
| 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
| 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.
| A search for nonthermal radio emission from OB and WR stars with RATAN-600
We have searched for nonthermal radio emission from 40 OB and WR stars.Enhanced nonthermal radio emission from an early-type star could beevidence for the presence of a collapsed companion, and thus for itsorigin as the result of a supernova explosion in a massive binary. Asshown in the evolutionary calculations of joint evolution of a neutronstar and a normal star in binaries (Lipunov & Prokhorov \cite{lp}),a considerable fraction of neutron stars in binary systems having anoptical companion must be in the ejector state. A neutron star in thisstate generates a relativistic wind like an isolated radio pulsar. Mostejectors in binary systems can not be identified as radio pulsarsbecause of absorption of radiowaves in the stellar wind of the normalcompanion, but instead, they may appear as sources of high-energy quantadue to the synchrotron radiation of relativistic particles (ejected bythe radio pulsar) in the magnetic field of a normal star (Lipunov &Prokhorov \cite{lp2}; Lipunov & Nazin \cite{naz}). In this case asource of nonthermal radiation in a wide range from radio to hardgamma-ray may appear as a result of a specific reflection effect in themagnetic field of the optical companion. Cyg X-3 and the periodicradioburster LS I +610303 may be examples of just this kind.To test this idea, measurements of radio flux densities in the rangefrom 0.96 to 21.7 GHz from selected OB and WR stars were made with theRATAN-600. No nonthermal radio emission from the selected stars weredetected.
| Abundances and Physical Conditions in the Warm Neutral Medium toward μ Columbae
We present ultraviolet interstellar absorption-line measurements for thesightline toward the O9.5 V star μ Columbae (l=237.3d, b=-27.1d d~400pc, z~180 pc; ~0.06 cm-3) obtained withthe Goddard High Resolution Spectrograph (GHRS) on board the HubbleSpace Telescope. These archival data represent the most complete GHRSinterstellar absorption-line measurements for any line of sight towardan early-type star. The 3.5 km s-1 resolution of theinstrument allows us to accurately derive the gas-phase column densitiesof many important ionic species in the diffuse warm neutral medium,including accounting for saturation effects in the data and forcontamination from ionized gas along this sightline. For thelow-velocity material (-20<~vLSR<~+15 kms-1), we use the apparent column density method to derivecolumn densities. For the individual absorbing components atvLSR~-28.8, +20.1, +31.0, and +41.2 km s-1, weapply component fitting techniques to derive column densities andb-values. We have also used observations of interstellar Lyαabsorption taken with the GHRS intermediate resolution gratings toaccurately derive the H I column density along this sightline. Theresulting interstellar column density, logN(H I)=19.86+/-0.015, is inagreement with other determinations but is significantly more precise.The low-velocity material shows gas-phase abundance patterns similar tothe warm cloud (cloud A) toward the disk star ζ Ophiuchi, while thecomponent at vLSR~+20.1 km s-1 shows gas-phaseabundances similar to those found in warm halo clouds. We find that thevelocity-integrated gas-phase abundances of Zn, P, and S relative to Halong this sightline are indistinguishable from solar system abundances.We discuss the implications of our gas-phase abundance measurements forthe composition of interstellar dust grains. We find a dust-phaseabundance [(Fe+Mg)/Si]d =2.7-3.3 in the low-velocity gas;therefore the dust cannot be composed solely of common silicate grains,but must also include oxides or pure iron grains. The low-velocitymaterial along this sightline is characterized by T~6000-7000 K withne~0.3 cm-3, derived from the ionizationequilibrium of Mg and Ca. The relative ionic column density ratios ofthe intermediate-velocity components at vLSR=+31.0 and +41.2km s-1 show the imprint both of elemental incorporation intograins and (photo)ionization. These clouds have low total hydrogencolumn densities [logN(H)~17.4-17.7], and our component fitting b-valuesconstrain the temperature in the highest velocity component to beT=4000+/-700 K. The electron density of this cloud is ne~0.6cm-3, derived from the 2P1/2 to2P3/2 fine structure excitation of C II. Thecomponents at vLSR~-30 and -48 km s-1 along thissightline likely trace shocked gas with very low hydrogen columndensities. The vLSR~-30 km s-1 component isdetected in a few strong low-ionization lines, while both are easilydetected in Si III. The relative column densities of the -30 kms-1 suggest that the gas is collisionally ionized at moderatetemperatures (T~25,000 K). This is consistent with the measured b-valuesof this component, though nonthermal motions likely contributesignificantly to the observed breadths.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| A common resonance of delta SCT stars and ellipsoidal binaries.
Not Available
| Diffuse ionized gas toward beta Canis Majoris
This paper presents the study of the interstellar medium toward betaCMa, a disk sight-line known for its low neutral gas density. This studyuses high and medium resolution HST-GHRS spectra including lines fromthe following species: H i, D i, N i, O i, S ii, S iii, Si ii, Si iii,Si iv, Al ii, Al iii, Fe ii, Mg i, Mg ii, Mn ii, C ii and C iv. The lineof sight to beta CMa (153 pc) is dominated by two ionized regions with avelocity difference of 10 km s(-1) . The ionized regions account formost of the total hydrogen column density, around 2 10(19) cm(-2) , andthe neutral gas represents only 10% of the total gas. The two ionizedclouds display characteristics of the warm diffuse gas detected in thedisk and the halo. Their gas-phase abundances indicate that theirdepletion is low, especially for the more ionized of the two clouds.Special models of photoionization by the two EUV-excess stars beta CMaand epsilon CMa would be needed for a detailed discussion of theionizing mechanisms of the clouds ; their ionization ratios arenevertheless roughly compatible with collisional ionization attemperatures around 20 000 K, substantially higher than the kinetictemperatures derived from the line widths. Their characteristics suggestthat the clouds may be in the process of cooling down and recombiningafter having been shocked and ionized by some violent events, possiblyrelated to the Local Bubble formation. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space TelescopeInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.
| On rotation of ellipsoidal binary systems.
Not Available
| 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.
| 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
| The z=0.558 absorption system towards PKS 0118-272: A candidate Damped LY α system at low redshift.
We present a study of the MgII absorption system at z=0.558 towards theBL Lac object PKS 0118-272 based on high resolution spectra(λ/{DELTA}λ=~2x10^4^) obtained at the 3.6m ESO telescopeand on direct imaging data obtained at the 3.5m ESO New TechnologyTelescope. At the redshift of the absorber we detect lines of the lowionization species MgI, MgII, CaII, TiII, MnII, and FeII. Most of thelines are unsaturated and we determine accurate column densities for allthe species but MgII. The derived column densities are typical of theinterstellar gas in the disk of the Galaxy. By assuming dust-free gaswith solar abundances these column densities yieldN(HI)>2.5x10^19^cm^-2^ however, the relative elemental abundancessuggest that some dust is present and that N(HI)=~2x10^20^cm^-2^. Theinferred HI column density indicates that the absorber is a DampedLyα system. We find [Ti/Fe]=+0.3, in agreement with [Ti/Fe]measurements in DLA absorbers, but [Mn/Fe]=+0.4, at variance with thevalues [Mn/Fe]<0 common to DLA systems. The measured [Ti/Fe] and[Mn/Fe] ratios match remarkably well the differential depletion patternof low-density interstellar clouds in the Galaxy. Unlike high-redshiftDLA systems (z>=2), the z=0.558 absorber seems to originate in agalaxy that has already attained the abundances and dust content ofpresent-day disk galaxies. The analysis of our imaging data lendssupport to the presence of an intervening galaxy. After a carefulsubtraction of the BL Lac image, an object at 1.6" from PKS 0118-272 isdetected. At the absorber redshift the projected distance of this closecompanion (14h_50_^-1^kpc) and its absolute magnitude (M_R_=~-22.3) areconsistent with those found for galaxies associated with low-redshiftDLA systems.
| New spectroscopic and photometric observations of the ellipsoidal variable V600 HER (HD 149881): solution of the light curves and the radial-velocity curve by the method of synthesis.
Not Available
| 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
| A Keck HIRES Investigation of the Metal Abundances and Kinematics of the Z = 2.46 Damped LY alpha System toward Q0201+365
We present high-resolution (~8 km s^-1^) spectra of the QSO Q0201+365obtained with HIRES, the echelle spectrograph on the 10 m W. M. KeckTelescope. Although we identify over 80% of the absorption features andanalyze several of the more complex metal-line systems, we focus ouranalysis on the damped Lyα system at z = 2.462. Ionizationsimulations suggest the hydrogen in this system is significantly neutraland all of the observed metals are predominantly singly ionized. Wemeasure accurate abundances for Fe, Cr, Si, and Ni and place a lowerlimit on the abundance of Zn: [Fe/H] = -0.830 +/- 0.051, [Cr/H] = -0.902+/- 0.064, [Si/H] = -0.376 +/- 0.052, [Ni/H] = -1.002 +/- 0.054, and[Zn/H] > -0.562 +/- 0.064. We give evidence suggesting the actual Znabundance is [Zn/H]~ -0.262, implying the highest metallicity observedat a redshift z > 2. The relative abundances of these elementsremains constant over essentially the entire system (~150 km s^-1^ invelocity space), suggesting it is well mixed. Furthermore, we use thelack of abundance variations to infer properties of the dust responsiblefor element depletion. Finally, we discuss the kinematic characteristicsof this damped Lyα system, comparing and contrasting it with othersystems. The low-ion line profiles span ~200 km s^-1^ in velocity spaceand have an asymmetric shape with the strongest feature on the red edge.These kinematic characteristics are consistent with a rotating diskmodel.
| 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
| 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
| Highly Ionized Interstellar Atoms--Heated, Cooled, or Mixed?
Recent observations with the Goddard High Resolution Spectrograph on theHubble Space Telescope, combined with Copernicus results, make possiblea comparison between C+3 and O+5 interstellar column densities in boththe halo and the disk of our Galaxy. The ratio N(C+3)/N(O+5) for sixlines of sight in the disk is about an order of magnitude less than forfive corresponding values in the halo. In the disk, the values of thisratio are in good general agreement with a variety of different modelsfor conductive heating at an interface between hot and cool gas. As aworking hypothesis we assume that this process is the dominant one forproducing the observed highly ionized species in the disk. The muchlarger ratios for the halo lie between the ranges predicted by twodifferent idealized models---radiative cooling of hot infalling gas andturbulent mixing of hot gas with the cool clouds past which it flows.Since both these processes should occur when hot gas flows past H Iclouds in the halo, we assume tentatively that these two processes maybe jointly responsible for much of the observed high ionization of haloatoms.
| 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
| Observations of Interstellar Clouds in the Galactic Halo
The vertical structure of the ISM in the Milky Way is complex. Across-sectional cut of the Galaxy would reveal a stratified distributionof material ranging from the cold (T =~ 10 K) dense molecular clouds,which are closely confined to the Galactic disk, to very hot (T =~ 10(6)K) diffuse gas extending into the Galactic halo with a scale height (h)of perhaps some 5 kpc. Between these extremes lie cold H I (theso-called ``standard clouds''; T =~ 100 K, h =~ 100 pc), warm H I clouds(T =~ 10(3-4) K, h =~ 500 pc), warm ionized H (T =~ 10(3-4) K, h =~ 1kpc), and hot gas (T =~ 10(5) K,h= 2--5 kpc). In order to understand thedynamical processes which produce and maintain this vertical structure--- and to understand the interrelationships among these distinctclasses of interstellar gas --- we must first investigate the physicalproperties of the observed phases of the ISM. Accurate measurements oftemperatures, compositions, ionization states, densities, etc., forindividual interstellar clouds are required for the development of aunified model of the ISM. The UV spectral region is rich inabsorption-line diagnostics which allow the study of interstellar matterover a wide range of physical conditions --- from the cold H I clouds,which produce strong absorption from many singly-ionized metals, to thehot 10(5) K gas seen in absorption from such ions as C(3+) and N(4+) .In this poster I will present results from a multiyear program of UVspectroscopy with the Goddard High Resolution Spectrograph aboard theHubble Space Telescope whose goal is to help characterise the propertiesof individual interstellar clouds in the Galactic disk and halo. I willconcentrate on results for three lines-of-sight extending into the lowerhalo towards the early-type, high-latitude stars HD 93521, HD 149881, HD215733 (z-distances =~ 1500 pc) and discuss the physical conditions inthe interstellar clouds and the implications for understanding theorigin of the structure of the ISM in the Galatic halo.
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Observation and Astrometry data
| Constellation: | Hercules |
| Right ascension: | 16h36m58.21s |
| Declination: | +14°28'30.9" |
| Apparent magnitude: | 7.015 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | -3.2 |
| Proper motion Dec: | 3.3 |
| B-T magnitude: | 6.793 |
| V-T magnitude: | 6.997 |
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INHABITED SKY
