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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.
| The standard theory of extinction and the spectrum of stars with very little reddening
This paper examines the relationship between spectra of stars of samespectral type with extremely low reddenings. According to the standardtheory, the relationship between the spectrum of stars with samespectral type and small, but different reddenings should be different inthe optical and in the UV. This difference is not observed: the ratio ofthe spectra of two stars in directions where the reddening is largeenough to be detected and low enough not to give a noticeable2200Å bump is an exponential of /1/λ from the near-infraredto the far-UV. This result is in conformity with the ideas introduced inpreceding papers: the exponential optical extinction extends to the UV,and the spectrum of stars with enough reddening is contaminated by lightscattered at close angular distance from the stars. An application willbe the determination of the spectrum of a non-reddened star from thespectrum of a star of same spectral type with little reddening.
| The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation
We used the Space Telescope Imaging Spectrograph (STIS) with itssmallest entrance aperture (0.03" wide slit) and highest resolutionechelle gratings (E140H and E230H) to record the interstellar absorptionfeatures for 10 different multiplets of neutral carbon at a resolvingpower of λ/Δλ=200,000 in the UV spectra of 21early-type stars. Our objective was to measure the amount of C I in eachof its three fine-structure levels of the ground electronic state, sothat we could determine the thermal pressures in the absorbing gas andhow much they vary in different regions. Our observations areprincipally along directions out to several kiloparsecs in the Galacticplane near longitudes l=120deg and 300°, with the moredistant stars penetrating nearby portions of the Perseus andSagittarius-Carina arms of the Galaxy. We devised a special analysistechnique to decipher the overlapping absorption features in thedifferent multiplets, each with different arrangements of the closelyspaced transitions. In order to derive internally consistent results forall multiplets, we found that we had to modify the relative transitionf-values in a way that made generally weak transitions stronger thanamounts indicated in the current literature. We compared our measuredrelative populations of the excited fine-structure levels to thoseexpected from equilibria calculated with collisional rate constants forvarious densities, temperatures, and compositions. The median thermalpressure for our entire sample was p/k=2240 cm-3 K, orslightly higher if the representative temperatures of the material aremuch above or below a most favorable temperature of 40 K for theexcitation of the first excited level at a given pressure. For gas thatis moving outside the range of radial velocities permitted bydifferential Galactic rotation between us and the targets, about 15% ofthe C I indicates a thermal pressure p/k>5000 cm-3 K. Forgas within the allowed velocities, this fraction is only 1.5%. Thiscontrast reveals a relationship between pressure enhancements and thekinematics of the gas. Regardless of velocity, we usually can registerthe presence of a very small proportion of the gas that seems to be atp/k>~105 cm-3 K. We interpret these ubiquitouswisps of high-pressure material to arise either from small-scale densityenhancements created by converging flows in a turbulent medium or fromwarm turbulent boundary layers on the surfaces of dense clouds movingthrough an intercloud medium. For turbulent compression, our C Iexcitations indicate that the barytropic indexγeff>~0.90 must apply if the unperturbed gas startsout with representative densities and temperatures n=10 cm-3and T=100 K. This value for γeff is larger than thatexpected for interstellar material that remains in thermal equilibriumafter it is compressed from the same initial n and T. However, ifregions of enhanced pressure reach a size smaller than ~0.01 pc, thedynamical time starts to become shorter than the cooling time, andγeff should start to approach the adiabatic valuecp/cv=5/3. Some of the excited C I may arise fromthe target stars' H II regions or by the effects of optical pumping fromthe submillimeter line radiation emitted by them. We argue that thesecontributions are small, and our comparisons of the velocities ofstrongly excited C I to those of excited Si II seem to support thisoutlook. For six stars in the survey, absorption features frominterstellar excited O I could be detected at velocities slightlyshifted from the persistent features of telluric origin. These O I* andO I** features were especially strong in the spectra of HD 93843 and HD210839, the same stars that show exceptionally large C I excitations. Inappendices of this paper, we present evidence that (1) the wavelengthresolving power of STIS in the E14OH mode is indeed about 200,000, and(2) the telluric O I* and O I** features exhibit some evidence formacroscopic motions, since their broadenings are in excess of thatexpected for thermal Doppler broadening at an exospheric temperatureT=1000 K. Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| The HB Narrowband Comet Filters: Standard Stars and Calibrations
We present results concerning the development and calibration of a newset of narrowband comet filters, designated the HB filter set, which wasdesigned and manufactured to replace aging IHW filters. Information isalso presented about the design and manufacturing of the filters,including the reasoning that was used for deciding the final wavelengthsand bandpasses. The new filters are designed to measure five differentgas species (OH, NH, CN, C2, C3), two ions(CO+, H2O+), and four continuum points.An improved understanding of extended wings from emission bands in cometspectra, gained since the development of the IHW filters, wasincorporated into the new design, so that contamination from undesiredspecies is significantly reduced compared to previous filters. Inaddition, advances in manufacturing techniques lead to squarertransmission profiles, higher peak transmission and UV filters withlonger lifetimes. We performed the necessary calibrations so that dataobtained with the filters can be converted to absolute fluxes, allowingfor, among other things, accurate subtraction of the continuum from thegas species. Flux standards and solar analogs were selected andobserved, and the data were used to establish a magnitude system for theHB filters. The star measurements were also used to evaluate which solaranalogs were best representatives of the Sun and to explore how the fluxstandards differed in the UV with respect to their spectral type. Newprocedures were developed to account for the non-linear extinction inthe OH filter, so that proper extrapolations to zero airmass can beperformed, and a new formalism, which can account for mutualcontaminations in two (or more) filters, was developed for reducingcomet observations. The relevant equations and reduction coefficientsare given, along with detailed instructions on how to apply them. Wealso performed a series of tests involving factors that can affecteither the filter transmission profiles or the distribution of theemission lines in the gas species to determine how these effectspropagate through to the calibration coefficients. The results indicatethat there are only two factors that are a concern at a level of morethan a few percent: f-ratios smaller than f/4, and a few individualfilters whose transmission profiles are significantly different from thefilters used in the calibrations.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| High-Resolution Ultraviolet Observations of the Highly Ionized Interstellar Gas toward Radio Loops I and IV
We present new Goddard High-Resolution Spectrograph (GHRS) echelleobservations of the high ionization lines of Si IV, C IV, and N V towardHD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernovaremnants. Absorption along the path to HD 119608 makes it possible tostudy energetic processes that may result in the flow of gas into theGalactic halo. The data have a resolution (FWHM) of ~3.5 km s-1 and S/Nratios of 30:1--50:1. The integrated high ion column densities log N =13.57 +/- 0.02, 14.48 +/- 0.06, and 13.45 +/- 0.07 for Si IV, C IV, andN V, respectively, imply a factor of 2--4 enhancement in the amount ofhighly ionized gas compared to average sight lines through the Galacticdisk and halo. The integrated high ion column density ratios, N(CIV)/N(Si IV) = 8.1 +/- 1.1 and N(C IV)/N(N V) = 10.7 +/- 2.1, are alsoseveral times larger than normal. These high ion results suggest theabsorption is influenced by passage of the sight line through the centerof Loop IV. The HD 119608 C IV absorption profile has a bimodal velocitystructure indicative of an expanding shell; we tentatively derive anexpansion velocity of 16 km s-1 for Radio Loop IV. A detailed analysisof the high ion profile structure indicates that multiple types ofhighly ionized gas with a range of properties exist along this sightline. We also reexamine the high ionization properties of the QSO 3C 273sight line using new intermediate-resolution (FWHM ~ 20 km s-1) GHRSdata. We obtain log N = 14.49 +/- 0.03 and 13.87 +/- 0.06 for C IV and NV, respectively. The C IV column density, which is 0.12 dex smaller thanearlier estimates, leads to somewhat smaller ionic ratios thanpreviously determined. We find N(C IV)/N(Si IV) = 5.1 +/- 0.6 and N(CIV)/N(N V) = 4.2 +/- 0.6. However, as for HD 119608, the high ion columndensities toward 3C 273 are larger than normal by factors of 2--4. The3C 273 high ion absorption profiles are much broader than those seentoward HD 119608 and other sight lines near the center of Loop IV. Thelarger line widths may result because the sight line passes through theturbulent edge of Loop IV as well as the X-ray and radio continuumemission regions of the North Polar Spur. We have compiled a list of thehighest quality IUE and GHRS high ion measurements available forinterstellar sight lines through the disk and halo and find thefollowing median averaged results: N(C IV)/N(Si IV) = 3.8 +/- 1.9 andN(C IV)/N(N V) = 4.0 +/- 2.4. These ratios are lower than those foundfor four Loop IV sight lines. We suggest a model for the production ofhighly ionized gas in Loop IV in which the contributions from turbulentmixing layers and conductive interfaces/SNR bubbles to the total highion column densities are approximately equal. Much of the high ionabsorption toward HD 119608 and 3C 273 may occur within a highlyfragmented medium within the remnant or the outer cavity walls of theremnant.
| The Composition of the Diffuse Interstellar Medium
Recent Goddard High Resolution Spectrograph measurements of Si, S, Cr,Mn, Fe, and Zn in interstellar clouds along lines of sight in theGalactic disk and into the lower halo are discussed. The gas-phaseabundance of S relative to H in the interstellar clouds appears to beindistinguishable from the solar value. For the other elements, we findwell-defined upper limits in the gas-phase abundances at significantlysubsolar values. For Fe, Mn, and Cr (and probably Ti), there are noconvincing cases in which the relative gas-phase abundances exceedroughly -0.5 dex, i.e., these elements are not seen in interstellar gaswith an abundance greater than about one-third solar. For Si, the limitis roughly -0.15 dex, and for Zn a constant abundance of -0.13 dex isfound from seven clouds along one halo sight line. These subsolarmaximum abundances have two possible interpretations: (1) they indicatethe presence of an essentially indestructible component of interstellardust, which contains about two-thirds of the Ti, Mn, Cr, and Fe andabout one-third of the Si (based on solar composition), or (2) theyindicate that the true total abundances of these elements aresubstantially less than in the Sun.
| 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
| 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.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| A 325 Square Degree Survey of B-Type Stars at High Galactic Latitudes
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...447..783L
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Secondary photometric standard stars for the Thuan-Gunn and Johnson-Kron-Cousins systems
CCD photometry is presented for 119 stars observed in the Thuan-Gunnphotometric system (the passbands vgr). The sample includes 82 stars forwhich magnitudes in this system have not previously been published.Photometry in the Johnson-Kron-Cousins photometric system (BV or UBV)has been obtained for a subsample of 85 of the stars. Of those starswhich are not primary standard stars for the Thuan-Gunn system 70observed in Gunn r, 48 observed in Gunn g, and 37 observed in Gunn vhave sufficient repeat measurements that they may be used as secondarystandard stars for CCD photometry. The typical uncertainties for themagnitudes of these stars are 0.006 mag.
| Physical Parameters of the Neutral Gas Coma of Halley's Comet 1986III on 1986MAY8-10
Not Available
| Photometry for Stars in the Field of V Bootis
Not Available
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| A high-resolution optical and radio study of Milky Way halo gas
Optical interstellar absorption lines of Ti II and Ca II and the 21 cmemission line of H I were observed at high-resolution (6 and 1 km/s,respectively) and high detection sensitivity along 25 lines of sight inthe Galactic halo. The sample includes 16 distant halo stars matchedwith one or more nearly aligned foreground stars as well as local starsalong five extragalactic sight lines. The data show substantialinterstellar material, at both low and intermediate velocities, between250 and 1000 pc beyond the Galactic plane. As much as one-third of thetotal gas observed in Ca II absorption may be beyond 1 kpc, and thegaseous Ti II may lie in an even thicker layer. The directly determinedgaseous Ti abundance above the Galactic plane exceeds that in the disk,on the average, by a factor of 4 to 6 and, for individual cloudcomponents, is further enhanced at higher LSR velocity. Thirty threediscrete high-latitude clouds are detected in Ca II absorption, and 17discrete clouds, including three high-velocity clouds, are identified inH I emission. The kinematics of the high-latitude gas observed in Ti IIand Ca II absorption is characterized by significant peculiar velocitieswith respect to a model corotating halo.
| CCD surface photometry for E and S0 galaxies in the Coma cluster
The paper presents CCD surface photometry in Johnson B and Gunn r for 33galaxies in the Coma cluster and for 13 galaxies in the field or inother clusters, obtained with the Danish 1.54-m telescope at the ESO, LaSilla, Chile in the period April 3-10, 1989. The data were calibratedwith standard stars. Data for all galaxies include derivations ofeffective radius, mean surface brightness, and total magnitude. Goodagreement was found between the photometry and the global parameterspresented with data of Dressler, Faber, Burstein and theircollaborators, indicating that the quality of the CCD data is fullyadequate for studies of the large-scale motions in the universe.
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
| The distribution of interstellar AL III away from the Galactic plane
IUE spectra are analyzed to study the density distribution ofinterstellar Al III away from the Galactic plane. In most cases, themeasured values of the relative line strengths are consistent with onlymodest levels of line saturation. Al III is found to have an exponentialscale height and 1 sigma errors of 1.02(+0.36, -0.24) kpc. For the sameset of 70 stars, the scale height and 1 sigma errors for H I are0.67(+0.21, -0.16) kpc. The Al III scale height is similar to the valueobtained for free electrons from pulsar dispersion measures. The ionizedgas traced by Al III is somewhat more extended than the neutral gastraced by H I but less extended than the very highly ionized gas tracedby Si IV, C IV, and N V.
| Runaway stars from young star clusters containing initial binaries. II - A mass spectrum and a binary energy spectrum
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99..608L&db_key=AST
| Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
| The kinematics of Milky Way halo gas. I - Observations of low-ionization species
Ultraviolet interstellar line day observed with the IUE toward 70 halostars and four extragalactic sight lines are analyzed in a study of thelarge-scale kinematic properties of the Milky Way halo gas. The motionsof the low-ionization gas is focused on. Large systematic velocities arefound, and a pronounced asymmetry in the absorption characteristics ofhalo gas toward the Galactic poles is indicated. In the north,substantial amounts of material are falling toward the disk atvelocities up to about 120 km/s in the most extreme case. Toward thesouth, low-ionization material shows no extreme or systematic motions.
| The density distribution of refractory elements away from the Galactic plane
The density distributions of the three refractory elements Ti II, Ca II,and Fe II away from the Galactic plane are compared with thedistribution of hydrogen and dust by examining plots of N s in b versusz. It is found that Ti II and Ca II are considerably more extended in zthan the H I and dust and that Fe II has an intermediate extension.Although the results are strongly influenced by sample bias, theindicated exponential scale heights for the data sample are h(Ti II) notless than 2 kpc, h(Ca II) = 1 kpc, h(Fe II) = 0.5 kpc, H(H I) = 0.3 kpc,and h(E/B-V) = 0.1 kpc. Furthermore, it is demonstrated that Ti II andCa II are much more smoothly distributed in space than the hydrogen ordust. The large scale heights for Ti II and Ca II and their smoothdistributions are most easily understood as the effect of a mixturealong the line of sight of two H I phases namely, a diffuse cloud phase,in which nearly all of the Ti and Ca are tied up in dust, and anintercloud medium, where refractory elements are less depleted. It isfound that Ti II and Ca II mostly trace the smoothly distributedintercloud medium. The smoothness of the distributions of Ti II and CaII makes them candidates for use as distance indicators.
| IUE-IRAS studies of the infrared cirrus
The 60 and 100 micron cirrus emission around 256 lines of sight in theIRAS all-sky survey was measured, and the flux averages were used tostudy the distribution, variations, and correlations of the IRASinfrared cirrus fluxes with various interstellar parameters. It wasfound that the 60 and 100 micron fluxes correlate with the depletion ofSi and show a trend with the depletion of Fe for 51 lines of sighttoward the Galactic halo. No correlation was found with the abundancesof Si, Mn, Fe, S, or Zn or with abundance ratios for the full sample of256 stars. An abundance ratio of about 3 x 10 to the 7th by numberrelative to H was derived from 60 and 100 micron flux ratios and the Hcolumn along the line of sight; this ratio appears to decrease by afactor of 10 into the halo.
| Galactic interstellar abundance surveys with IUE. III - Silicon, manganese, iron, sulfur, and zinc
This paper continues a survey of intestellar densities, abundances, andcloud structure in the Galaxy using the IUE satellite. A statisticaldata set of 223 O3-B2.5 stars is constructed, including 53 stars in theGalactic halo. It is found that S II lines in B stars, of luminosityclasses IV and V, have possible contamination from stellar S II,particular for stars with v sin i less than 200 km/s. The meanlogarithmic depletions are -1.00, -1.19. -0.63, and -0.23 (Si, Mn,Fe,S,Zn). Depletions of Si, Mn, and Fe correlate with the mean hydrogendensity n-bar along the line of sight, with a turnover for n-bar greaterthan 1/cm. Sulfur depletions correlate with n-bar along the line ofsight. The slight Zn depletion correlation also appears to bestatistically insignificant. No correlation of depletion is found withthe physical density derived from H2 rotational states in 21 lines ofsight. Depletion variations in the disk are consistent with a Galacticabundance gradient or with enhanced mean depletions in the anticenterregion.
| Galactic interstellar abundance surveys with IUE. II - The equivalent widths and column densities
This paper continues a survey of interstellar densities, abundances, andcloud structure in the Galaxy, using the International UltravioletExplorer (IUE) satellite. Equivalent widths of 18 ultraviolet resonancetransitions are presented and column densities for Si II, Mn II, Fe II,S II, and Zn II toward 261 early-type stars are derived. Theseequivalent widths and column densities agree within the stated errors ofearlier Copernicus, BUSS, or IUE surveys of Mn II, Fe II, S II, and ZnII for 45 stars in common. The column densities are derived fromsingle-component curves of growth with a common b-value based on that ofFe II and Si II.
| The galactic corona
The IUE observations and theories involving the gaseous galactic coronaof the Milky Way are reviewed. Observational topics emphasized includeresults relevant to the z distribution, kinematics, and ionic andelemental abundances of the gas. The theories for the support,ionization, and kinematics of the galactic halo appear to require amerging of the ideas associated with the galactic fountain models andthe photoionized halo models. The Si IV found in the galactic halo islikely mostly produced by photoionization while the N V is probablymostly produced by collisional ionization in cooling fountain gas; C IVmay represent an intermediate case with important contributions fromphotoionization and from collisional ionization.
| The extent of the local H I halo
Forty-five high-latitude, OB stars have been observed in the Ly alphaand 21 cm lines of HI in an effort to map out the vertical distributionand extent of the local HI halo. The 25 stars for which a reliable HIcolumn density can be obtained from Ly alpha lie between 60 and 3100 pcfrom the plane. The principal result is that the total column density ofHI at an absolute value of z greater than 1 kpc is, on the average, 5 +or - 3 x 10 to the 19th power/sq cm, or 15 percent of thetotalHI. At relatively low z the data toward some starssuggest a low effective scale height and fairly high average foregrounddensity, while toward others the effective scale height is large and theaverage density is low. This can be understood as the result ofirregularities in the interstellar medium. A model with half of the HImass in clouds having radii of a few pc and a Gaussian verticaldistribution with sigma2 = 135 pc, and half of the mass in anexponential component with a scale height of 500 pc, gives asatisfactory fit to the data. The technique of comparing Ly alpha and 21cm column densities is also used to discuss the problem of estimatingthe distance to several possibly subluminous stars.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | בתולה |
| התרוממות ימנית: | 13h47m19.20s |
| סירוב: | -02°26'36.6" |
| גודל גלוי: | 7.86 |
| מרחק: | 298.507 פארסק |
| תנועה נכונה: | -0.9 |
| תנועה נכונה: | -1.5 |
| B-T magnitude: | 7.633 |
| V-T magnitude: | 7.842 |
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