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Resim Yükleyin
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Ýlgili Makaleler
A multiwavelength investigation of the temperature of the cold neutral medium
We present measurements of the HI spin temperatures (Ts) ofthe cold neutral medium (CNM) towards radio sources that are closelyaligned with stars for which published H2 ortho-paratemperatures (T01) are available from ultraviolet (UV)observations. Our sample consists of 18 radio sources close to 16 nearbystars. The transverse separation of the lines of sight of thecorresponding UV and radio observations varies from 0.1 to 12.0 pc atthe distance of the star. The UV measurements do not have velocityinformation, so we use the velocities of low ionization species (e.g.NaI/KI/CI) observed towards these same stars to make a plausibleidentification of the CNM corresponding to the H2 absorption.We then find that T01 and Ts match withinobservational uncertainties for lines of sight with H2 columndensity above 1015.8cm-2, but deviate from eachother below this threshold. This is consistent with the expectation thatin the CNM Ts tracks the kinetic temperature due tocollisions and that T01 is driven towards the kinetictemperature by proton exchange reactions.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| Winds from OB Stars: A Two-Component Scenario?
X-ray spectroscopy of several OB stars with massive winds has revealedthat many X-ray line profiles exhibit unexpectedly small blueshifts andare almost symmetric. Moreover, the hottest X-ray lines appear tooriginate closest to the star. These properties appear to beinconsistent with the standard model of X-rays originating in shockedmaterial in line-driven spherically symmetric winds. Here we raise thequestion, can the X-ray line data be understood in terms of atwo-component wind? We consider a scenario in which one component of thewind is a standard line-driven wind that emerges from a broad range oflatitudes centered on the equator. The second component of the windemerges from magnetically active regions in extensive polar caps. Theexistence of such polar caps is suggested by a recent model of dynamoaction in massive stars. We describe how the two-component model isconsistent with a variety of observational properties of OB star winds.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| B-type supergiants in the Small Magellanic Cloud: rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants havebeen analysed to estimate the contributions of both macroturbulence androtation to the broadening of their metal lines. Two differentmethodologies are considered, viz. goodness-of-fit comparisons betweenobserved and theoretical line profiles and identifying zeros in theFourier transforms of the observed profiles. The advantages andlimitations of the two methods are briefly discussed with the lattertechniques being adopted for estimating projected rotational velocities(v sin i) but the former being used to estimate macroturbulentvelocities. The projected rotational velocity estimates range fromapproximately 20 to 60 km s-1, apart from one SMC supergiant,Sk 191, with a v sin i ≃ 90 km s-1. Apart from Sk 191,the distribution of projected rotational velocities as a function ofspectral type are similar in both our Galactic and SMC samples withlarger values being found at earlier spectral types. There is marginalevidence for the projected rotational velocities in the SMC being higherthan those in the Galactic targets but any differences are only of theorder of 5-10 km s-1, whilst evolutionary models predictdifferences in this effective temperature range of typically 20 to 70 kms-1. The combined sample is consistent with a linearvariation of projected rotational velocity with effective temperature,which would imply rotational velocities for supergiants of 70 kms-1 at an effective temperature of 28 000 K (approximately B0spectral type) decreasing to 32 km s-1 at 12 000 K (B8spectral type). For all targets, the macroturbulent broadening wouldappear to be consistent with a Gaussian distribution (although otherdistributions cannot be discounted) with an 1/e half-width varying fromapproximately 20 km s-1 at B8 to 60 km s-1 at B0spectral types.
| Physical parameters and wind properties of galactic early B supergiants
We present optical studies of the physical and wind properties, plus CNOchemical abundances, of 25 O9.5-B3 Galactic supergiants. We employnon-LTE, line blanketed, extended model atmospheres, which provide amodest downward revision in the effective temperature scale of early Bsupergiants of up to 1-2 kK relative to previous non-blanketed results.The so-called "bistability jump" at B1 (Teff 21 kK)from Lamers et al. is rather a more gradual trend (with large scatter)from v&infy;/vesc3.4 for B0-0.5 supergiantsabove 24 kK to v&infy;/vesc 2.5 for B0.7-1supergiants with 20 kK ≤ Teff ≤ 24 kK, andv&infy;/vesc 1.9 for B1.5-3 supergiants below20 kK. This, in part, explains the break in observed UV spectralcharacteristics between B0.5 and B0.7 subtypes as discussed by Walbornet al. We compare derived (homogeneous) wind densities with recentresults for Magellanic Cloud B supergiants and generally confirmtheoretical expectations for stronger winds amongst Galacticsupergiants. However, winds are substantially weaker than predictionsfrom current radiatively driven wind theory, especially at mid-Bsubtypes, a problem which is exacerbated if winds are already clumped inthe Hα line forming region. In general, CNO elemental abundancesreveal strongly processed material at the surface of Galactic Bsupergiants, with mean N/C and N/O abundances 10 and 5 times higher thanthe Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating thelowest degree of processing in our sample, and HD 152236 (B1.5Ia+) the highest.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| Understanding B-type supergiants in the low metallicity environment of the SMC II
Despite a resurgence of effort over the last decade in the area ofmassive stars there is still ambiguity over their evolutionary path,contamination of their surface abundances and the behaviour of theirstellar winds. Here 10 SMC B-type supergiants are analysed applying aunified model atmosphere code fastwind to intermediate resolutionspectra from the ESO Multi Mode Instrument (emmi) on the NTT telescope.Combined with the 8 targets analysed in Paper I (Trundle et al. 2004),this work provides observational results on the properties of the windsand chemical compositions of B-type supergiants in the SMC. This paperemphasizes and substantiates the implications for stellar evolution fromPaper I; that current theoretical models need to produce larger degreesof surface nitrogen enhancements at lower rotational velocities. Inaddition a significant discrepancy between theoretical and observedmass-loss rates is discussed which will have important implications forthe rotational velocities obtained from stellar evolution calculations.Furthermore, an initial calibration of the wind-momentum luminosityrelationship for B-type supergiants in a low metallicity environment (Z= 0.004) is presented.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| The Ultraviolet and Optical Spectra of Luminous B-Type Stars in the Small Magellanic Cloud
We present ultraviolet spectra from the Space Telescope ImagingSpectrograph (STIS) of 12 early B-type stars in the Small MagellanicCloud (SMC), composed of nine supergiants and three giants. Amorphological comparison with Galactic analogs is made using archivaldata from the International Ultraviolet Explorer (IUE). In general, theintensity of the P Cygni emission in the UV resonance lines is greaterand is seen to later spectral types in the Galactic spectra than intheir metal-deficient SMC counterparts. We attribute these effects asmost likely arising from weaker stellar winds in the SMC targets, aspredicted by radiatively driven wind theory. We also include unpublishedSTIS observations of two late O-type stars in the SMC. In combinationwith published O-type STIS data, we now have an extensive ultravioletspectral library of metal-deficient stars to use in the study ofunresolved starbursts and high-redshift star-forming galaxies. In thiscontext, we present empirical measurements for the B-type spectra of thenew ``1978 index'' suggested by Rix et al. as a probe of metallicity insuch systems.
| Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.
| Video observation of meteors at Yunnan Observatory
In the last 20 years, with the development of the CCD and imageintensifiers, the use of small flexible video meteor observation systemshas gradually increased, with the prospect that one day videoobservation will replace the visual observation and ordinaryphotographic observations. In this paper we report on the research anddevelopment of the No.1 meteor-comet video camera system of YunnanObservatory and some preliminary observed results. The system consistsof 5 changeable modules; it has a 36° large-field camera dedicatedto the observation of meteors, with which a magnitude 6 star can berecorded on a single frame with an accuracy of about 0.2 mag. We alsopresent a comparison of the video camera system with the traditionalphotographic system, and outline the merits, possible improvements andfuture development of the video system.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Terminal Velocities of Luminous, Early-Type Stars in the Small Magellanic Cloud
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS)are used to determine terminal velocities for 11 O and B-type giants andsupergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IVresonance lines. Using archival data from observations with the GoddardHigh-Resolution Spectrograph and the International Ultraviolet Explorertelescope, terminal velocities are obtained for a further five B-typesupergiants. We discuss the metallicity dependence of stellar terminalvelocities for supergiants, finding no evidence for a significantscaling between Galactic and SMC metallicities forTeff<30,000 K, consistent with the predictions ofradiation-driven wind theory. A comparison of thev&infy;/vesc ratio between the SMC and Galacticsamples, while consistent with the above statement, emphasizes that theuncertainties in the distances to galactic OB-type stars are a seriousobstacle to a detailed comparison with theory. For the SMC sample thereis considerable scatter in v&infy;/vesc at agiven effective temperature, perhaps indicative of uncertainties instellar masses.
| Interstellar Carbon in Translucent Sight Lines
We report interstellar C II column densities or upper limits determinedfrom weak absorption of the 2325.4029 Å intersystem transitionobserved in six translucent sight lines (AV>~1) with theSpace Telescope Imaging Spectrograph (STIS). The sight lines sample awide range of interstellar characteristics, including total to selectiveextinction, RV=2.6-5.1 average hydrogen density along thesight line, =3-14 cm-3 and fraction of H inmolecular form, 0-~40%. Four of the sight lines, those toward HD 37021,HD 37061, HD 147888, and HD 207198, have interstellar gas-phaseabundances that are consistent with the diffuse sight line ratio of161+/-17 carbon atoms in the gas per million hydrogen nuclei. We notethat while it has a gas-phase carbon abundance that is consistent withthe other sight lines, a large fraction of the C II toward HD 37061 isin an excited state. The sight line toward HD 152590 has a measuredinterstellar gas-phase carbon abundance that is well above the diffusesight line average; the column density of C in this sight line may beoverestimated because of noise structure in the data. Toward HD 27778 wefind a 3 σ abundance upper limit of <=108 C atoms in the gasper million H, a substantially enhanced depletion of C as compared tothe diffuse sight line value. The interstellar characteristics toward HD27778 are otherwise not extreme among the sample, except for anunusually large abundance of CO molecules in the gas.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Photospheric and stellar wind variability in ɛ Ori (B0 Ia)
We provide direct observational evidence for a link between photosphericactivity and perturbations in the dense inner-most stellar wind regionsof the B supergiant star ɛ Ori. The results, which are relevantto our understanding of the origin of wind structure, are based on amulti-spectral line analysis of optical time-series data secured in 1998using the HEROS spectrograph on the ESO Dutch 0.9-m telescope in LaSilla. A period of 1.9 days is consistently identified in Balmer,He I absorption, and weak metal lines such as Si III and C II. Theprimary characteristic is a large-amplitude swaying of the centralabsorption trough of the line, with differential velocities in linesformed at varying depths in the atmosphere. The variance resulting fromthe ``S-wave'' velocity behaviour of the lines is constrained within± the projected rotation velocity (80 km s-1) inthe weakest absorption lines, but extends blue-ward to over -200 kms-1 in Hα. A second (superimposed) 1.9 day signal ispresent at more extended blue-ward velocities (to -300 kms-1) in lines containing stronger circumstellar components.Inspection of archival optical data from 1996 provides evidence thatthis modulation signal has persisted for at least 2.5 years. Non-radialpulsational modelling is carried out in an attempt to reproduce the keyobservational characteristics of the line profile variability. Onlylimited success is obtained with prograde (m=-1) modes. The principalS-wave pattern cannot be matched by these models and remains enigmatic.Based on observations obtained as part of the MUSICOS 98 campaign fromESO La Silla, Chile.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS-32985.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| Macroturbulent and rotational broadening in the spectra of B-type supergiants
The absorption-line spectra of early B-type supergiants show significantbroadening that implies that an additional broadening mechanism(characterized here as `macroturbulence') is present in addition torotational broadening. Using high-resolution spectra withsignal-to-noise ratios of typically 500, we have attempted to quantifythe relative contributions of rotation and macroturbulence, but evenwith data of this quality significant problems were encountered.However, for all our targets, a model where macroturbulence dominatesand rotation is negligible is acceptable; the reverse scenario leads topoor agreement between theory and observation. Additionally, there ismarginal evidence for the degree of broadening increasing with linestrength, possibly a result of the stronger lines being formed higher inthe atmosphere. Acceptable values of the projected rotational velocityare normally less than or equal to 50 km s-1, which may alsobe a typical upper limit for the rotational velocity. Our best estimatesfor the projected rotational velocity are typically 10-20 kms-1 and hence compatible with this limit. These values arecompared with those predicted by single star evolutionary models, whichare initially rapidly rotating. It is concluded that either these modelsunderestimate the rate of rotational breaking or some of the targets maybe evolving through a blue loop or are binaries.
| The latitude and epoch for the formation of the southern Greek constellations
Not Available
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Chemical composition of B-type supergiants in the OB 8, OB 10, OB 48, OB 78 associations of M 31
Absolute and differential chemical abundances are presented for thelargest group of massive stars in M 31 studied to date. These resultswere derived from intermediate resolution spectra of seven B-typesupergiants, lying within four OB associations covering a galactocentricdistance of 5-12 kpc. The results are mainly based on an LTE analysis,and we additionally present a full non-LTE, unified model atmosphereanalysis of one star (OB 78-277) to demonstrate the reliability of thedifferential LTE technique. A comparison of the stellar oxygen abundancewith that of previous nebular results shows that there is an offset ofbetween ~ 0.15-0.4 dex between the two methods which is criticallydependent on the empirical calibration adopted for the R_23 parameterwith [O/H]. However within the typical errors of the stellar and nebularanalyses (and given the strength of dependence of the nebular results onthe calibration used) the oxygen abundances determined in each methodare fairly consistent. We determine the radial oxygen abundance gradientfrom these stars, and do not detect any systematic gradient across thisgalactocentric range. We find that the inner regions of M 31 are not, aspreviously thought, very ``metal rich''. Our abundances of C, N, O, Mg,Si, Al, S and Fe in the M 31 supergiants are very similar to those ofmassive stars in the solar neighbourhood.
| An analysis of STIS HST UV spectra of M 33 early B supergiants
We present terminal velocities of M 33 B-supergiants, obtained from STISHST spectra as part of our programme to investigate the Wind Momentum -Luminosity Relationship (WLR) in the Local Group. Terminal velocitiesare derived from their N V, C Iv, and Si Iv resonance lines in UVspectra. Comparing with IUE spectra of Galactic B-supergiants we foundevidence of low metallicity in three of our objects. The terminalvelocities are consistent with the corresponding values of Galacticstars, except for B-133. For this star we find a very largevinfty and a red Si Iv component deeper than the blue one,that might be an indication of binarity. The average ratio betweenterminal and turbulent wind velocities is 0.25, well above the valuefound for Galactic stars. Partly based on INES data from the IUEsatellite.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| Radio and submillimetre observations of epsilon Ori
In common with other early-type stars, epsilon Ori (B0 Ia) showsevidence for structure in its stellar wind. Variations in optical andultraviolet line profiles reveal the presence of large-scale structurein the inner wind. The detection of X-rays and the existence of blacktroughs in saturated ultraviolet lines are indicative of small-scalestructure. The geometric extent of both types of structure is poorlyknown. In principle, large-scale structure can be detected directly fromvery high spatial resolution observations that resolve the stellar wind.A simpler technique is to look for the presence of additional fluxcompared to that expected from a smooth wind. The run of this excessflux as a function of wavelength indicates how fast structure decays inthe wind. If there is variability in the excess flux, it shows us thatthe structure must be large-scale. Such variability is suggested by twoprevious 6 cm radio observations of epsilon Ori: Abbott et al.(\cite{Abbott+al80}) found 1.6 +/- 0.5 mJy, while Scuderi et al.(\cite{Scuderi+al98}) measured only 0.60 +/- 0.06 mJy. This couldindicate that the large-scale structure persists beyond ~ 50R*. To further investigate this variability, we used the VeryLarge Array (VLA) to monitor epsilon Ori over a 5-day period in February1999. We supplemented our data with observations from the VLA archive.In an attempt to resolve the stellar wind, we also obtained a series ofhigh spatial resolution observations with the Multi-Element Radio LinkedInterferometer Network (MERLIN) during January-March 1999. From thiscombined material we find no evidence for variability and we concludethat the Abbott et al. (\cite{Abbott+al80}) flux determination is inerror. The data do show substantial excess flux at millimetrewavelengths, compared to a smooth wind. This excess is confirmed by asubmillimetre observation which we obtained with the James Clerk MaxwellTelescope (JCMT). The behaviour of epsilon Ori is therefore similar towhat had been found previously for alpha Cam,delta Ori A, kappa Ori andzeta Pup. While the present data do not allow verystrong constraints, they show that considerable structure must persistup to at least ~ 10 R* in the wind of epsilon Ori. Thecombined radio fluxes are used to derive a mass-loss rate of log dot {M}(Msun/yr)= -5.73 +/- 0.04. This value is in good agreementwith the Hα mass-loss rate. The good agreement between Hαand radio mass-loss rates for hot stars in general remains puzzling, asit implies that the same amount of structure is present in verydifferent formation regions.
| 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.
| Spectroscopic Classification of 42 Large Magellanic Cloud OB Stars: Selection of Probes for the Hot Gaseous Halo of the Large Magellanic Cloud
Interstellar C IV absorption-line studies of the hot gaseous halo of theLarge Magellanic Cloud (LMC) have been hindered by nonideal selectionsof early-type probe stars in regions where C+3 can beproduced locally via photoionization, fast stellar winds, or supernovae.To observe stars outside such regions, precise spectral classificationsof OB stars in the field are needed. Therefore, we have obtainedmedium-dispersion spectra of 42 early-type stars in the LMC that aredistributed outside superbubbles or supergiant shells. The spectralclassification of these stars is presented in this paper. Nineteen ofthese program stars have spectral types between B1 and O7 and are thussuitable probes for interstellar C IV absorption-line studies of the hotgaseous halo of the LMC.
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