Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| Multi-periodic photospheric pulsations and connected wind structures in HD 64760
We report on the results of an extended optical spectroscopic monitoringcampaign on the early-type B supergiant HD 64760(B0.5 Ib) designed to probe the deep-seated origin of spatial windstructure in massive stars. This new study is based on high-resolutionechelle spectra obtained with the Feros instrument at ESO La Silla. 279spectra were collected over 10 nights between February 14 and 24, 2003.From the period analysis of the line-profile variability of thephotospheric lines we identify three closely spaced periods around 4.810h and a splitting of ±3%. The velocity - phase diagrams of theline-profile variations for the distinct periods reveal characteristicprograde non-radial pulsation patterns of high order corresponding topulsation modes with l and m in the range 6{-}10. A detailed modeling ofthe multi-periodic non-radial pulsations with the Bruce and Kyliepulsation-model codes (Townsend 1997b, MNRAS, 284, 839) favors eitherthree modes with l=-m and l=8,6,8 or m=-6 and l=8,6,10 with the secondcase maintaining the closely spaced periods in the co-rotating frame.The pulsation models predict photometric variations of 0.012{-}0.020 magconsistent with the non-detection of any of the spectroscopic periods byphotometry. The three pulsation modes have periods clearly shorter thanthe characteristic pulsation time scale and show small horizontalvelocity fields and hence are identified as p-modes. The beating of thethree pulsation modes leads to a retrograde beat pattern with tworegions of constructive interference diametrically opposite on thestellar surface and a beat period of 162.8 h (6.8 days). This beatpattern is directly observed in the spectroscopic time series of thephotospheric lines. The wind-sensitive lines display features ofenhanced emission, which appear to follow the maxima of the photosphericbeat pattern. The pulsation models predict for the two regionsnormalized flux amplitudes of A=+0.33,-0.28, sufficiently large to raisespiral co-rotating interaction regions (Cranmer & Owocki 1996, ApJ,462, 469). We further investigate the observed Hα wind-profilevariations with a simple rotating wind model with wind-densitymodulations to simulate the effect of possible streak lines originatingfrom the localized surface spots created by the NRP beat pattern. It isfound that such a simple scenario can explain the time scales and somebut not all characteristics of the observed Hα line-profilevariations.
| 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.
| Reanalysis of Copernicus Measurements of Interstellar Carbon Monoxide
We used archival data acquired with Copernicus to reexamine CO columndensities, as self-consistent oscillator strengths are now available.Our focus is on lines of sight containing modest amounts of molecularspecies. Our resulting column densities are small enough thatself-shielding from photodissociation does not occur in the cloudsprobed by the observations. While our sample shows that the columndensities of CO and H2 are related, no correspondence withthe CH column density is evident. The case for the CH+ columndensity is less clear. Recent chemical models for these sight linessuggest that CH is mainly a by-product of CH+ synthesis inlow-density gas. The models are most successful in reproducing theamounts of CO in the densest sight lines. Thus, much of the COabsorption must arise from denser clumps along the line of sight toaccount for the trend with H2.
| 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 total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| A search for O VI in the winds of B-type stars
We have conducted a survey of FUSE spectra of 235 Galactic B-type starsin order to determine the boundaries in the H-R diagram for theproduction of the superion {O Vi} in their winds. By comparing thelocations and morphology of otherwise unidentified absorption featuresin the vicinity of the {O Vi} resonance doublet with the bona fide windprofiles seen in archival IUE spectra of the resonance lines of {N V},{Si Iv} and {C Iv}, we were able to detect blueshifted {O Vi} lines inthe spectra of giant and supergiant stars with temperature classes aslate as B1. No features attributable to {O Vi} were detected in dwarfslater than B0, or in stars of any luminosity class later than B1,although our ability to recognize weak absorption features in thesestars is severely restricted by blending with photospheric andinterstellar features. We discuss evidence that the ratio of the ionfractions of {O Vi} and {N V} is substantially different in the winds ofearly B-type stars than O-type stars.
| The latitude and epoch for the formation of the southern Greek constellations
Not Available
| Wind variability of B supergiants. IV. A survey of IUE time-series data of 11 B0 to B3 stars
We present the most suitable data sets available in the InternationalUltraviolet Explorer (IUE) archive for the study of time-dependentstellar winds in early B supergiants. The UV line profile variability in11 B0 to B3 stars is analysed, compared and discussed, based on 16separate data sets comprising over 600 homogeneously reducedhigh-resolution spectrograms. The targets include ``normal'' stars withmoderate rotation rates and examples of rapid rotators. A gallery ofgrey-scale images (dynamic spectra) is presented, which demonstrates therichness and range of wind variability and highlights differentstructures in the winds of these stars. This work emphasises thesuitability of B supergiants for wind studies, under-pinned by the factthat they exhibit unsaturated wind lines for a wide range of ionization.The wind activity of B supergiants is substantial and has highly variedcharacteristics. The variability evident in individual stars isclassified and described in terms of discrete absorption components,spontaneous absorption, bowed structures, recurrence, and ionizationvariability and stratification. Similar structures can occur in stars ofdifferent fundamental parameters, but also different structures mayoccur in the same star at a given epoch. We discuss the physicalphenomena that may be associated with the spectral signatures. Thediversity of wind patterns evident likely reflects the role of stellarrotation and viewing angle in determining the observationalcharacteristics of azimuthally extended structure rooted at the stellarsurface. In addition, SEI line-synthesis modelling of the UV wind linesis used to provide further information about the state of the winds inour program stars. Typically the range, implied by the line profilevariability, in the product of mass-loss rate and ion fraction (mdotq_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ;it can however be several times larger over localised velocity regions.At a given effective temperature the mean relative ion ratios can differby a factor of 5. The general excess in predicted (forward-scattered)emission in the low velocity regime is discussed in terms of structuredoutflows. Mean ion fractions are estimated over the B0 to B1 spectralclasses, and trends in the ionic ratios as a function of wind velocityare described. The low values obtained for the ion fractions of UVresonance lines may reflect the role of clumping in the wind.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| Multicomponent radiatively driven stellar winds. II. Gayley-Owocki heating in multitemperature winds of OB stars
We show that the so-called Gayley-Owocki (Doppler) heating is importantfor the temperature structure of the wind of main sequence stars coolerthan the spectral type O6. The formula for Gayley-Owocki heating isderived directly from the Boltzmann equation as a direct consequence ofthe dependence of the driving force on the velocity gradient. SinceGayley-Owocki heating deposits heat directly on the absorbing ions, wealso investigated the possibility that individual components of theradiatively driven stellar wind have different temperatures. This effectis negligible in the wind of O stars, whereas a significant temperaturedifference takes place in the winds of main sequence B stars for starscooler than B2. Typical temperature differences between absorbing ionsand other flow components for such stars is of the order 103K. However, in the case when the passive component falls back onto thestar, the absorbing component reaches temperatures of order106 K, which allows for emission of X-rays. Moreover, wecompare our computed terminal velocities with the observed ones. Wefound quite good agreement between predicted and observed terminalvelocities. The systematic difference coming from the using of the socalled ``cooking formula'' has been removed.
| 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
| Dating Ptolemy's star catalogue through proper motions : the Hipparchan epoch.
Not Available
| The Normal Energy Distributions in Stellar Spectra: Giants and Supergiants
We have derived the normal spectral energy distributions for thoseearly-type subgiants, giants, and supergiants that were not investigatedin our earlier studies, which were in most cases also not included inthe studies of Sviderskiene. Color indices computed using our normalenergy distributions are in good agreement with normal colors derivedfrom observations in the Vilnius photometric system. The reliability ofour distribution curves is also demonstrated by comparisons of observedand computed (W-B)-(B-V) two-color diagrams in the WBVR system. Normalcolor indices for the photometric WBVR system are derived.
| An Empirical Test of the Mg II λ1240 Doublet Branching Ratio and Oscillator Strength
We empirically confirm the theoretical branching ratio and oscillatorstrength for the weak Mg II 3s-4p doublet at 1240 Å as determinedby Theodosiou & Federman. We use the independent methods of apparentoptical depth analysis for the sight lines toward μ Col, γ Ara,and ρ Leo and profile component fitting for the sight line towardμ Col in order to determine the branching ratio. We findf1239/f1240=1.74+/-0.06, in agreement with thetheoretical value of 1.78+/-0.03. Profile fitting for the line of sighttoward μ Col gives an f-value for the doublet of9.71+/-0.32x10-4, which agrees with both the theoreticalvalue of Theodosiou & Federman and the empirical value ofFitzpatrick. Based on observations obtained with the NASA/ESA HubbleSpace Telescope through the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NASA-26555.
| Repetitive structure in the stellar wind of HD 93843: a normal O-type star
We present the results from a 28-day IUE time-series campaign monitoringthe stellar wind of the O5-type giant HD 93843. The principal aim was tostudy variability in the wind of a star with a normal projected rotationvelocity. Systematic changes are identified, amidst continuousline-profile variability, in the absorption troughs of the Siiv and Nvresonance lines. The patterns observed have characteristic time-scalesof several days and are mimicked by fluctuations (of several 100 kms^-1) in the blue wings of the saturated Civ P Cygni profile. Fourieranalysis provides support for the repeatability of wind structures in HD93843 on a 7.1-d `period'. Power at this frequency is evident only atintermediate and high velocities (i.e., above ~0.3 of the terminalvelocity). The long modulation time-scale suggests that changes in thestar itself probably provide the physical source for triggering theonset of wind structure. Unfortunately the rotational, photometric,pulsational and magnetic properties of HD 93843 are too poorlyconstrained or known to permit a more detailed interpretation of the7.1-d wind modulation in terms of potential inhomogeneities at thestellar surface. Nevertheless, our study demonstrates that the incidenceof cyclic, possibly regular, stellar-wind variability is not restrictedto rapid rotators. Comparisons with other OB stars which have exhibitedrepetitive wind changes on `periods' of several days suggest that thetime-dependent UV properties of HD 93843 are more akin to those of theO4-type supergiant zeta Puppis.
| The Definitive Abundance of Interstellar Oxygen
Using the Goddard High Resolution Spectrograph (GHRS) onboard the HubbleSpace Telescope, we have obtained high signal-to-noise (S/N) ratioechelle observations of the weak interstellar O I lambda 1356 absorptiontoward the stars gamma Cas, epsilon Per, delta Ori, epsilon Ori, 15 Mon,tau CMa, and gamma Ara. In combination with previous GHRS measurementsin six other sight lines ( zeta Per, xi Per, lambda Ori, < Ori, kappaOri, and zeta Oph), these new observations yield a mean interstellargas-phase oxygen abundance (per 106 H atoms) of 106 O/H = 319 +/- 14.The largest deviation from the mean is less than 18%, and there are nostatistically significant variations in the measured O abundances fromsight line to sight line and no evidence of density-dependent oxygendepletion from the gas phase. Assuming various mixtures of silicates andoxides, the abundance of interstellar oxygen tied up in dust grains isunlikely to surpass 106 O/H ~ 180. Consequently, the GHRS observationsimply that the total abundance of interstellar oxygen (gas plus grains)is homogeneous in the vicinity of the Sun and about two-thirds of thesolar value of 106 O/H = 741 +/- 130. This oxygen deficit is consistentwith that observed in nearby B stars and similar to that recently foundfor interstellar krypton with GHRS. Possible explanations for thisdeficit include: (1) early solar system enrichment by a local supernova,(2) a recent infall of metal-poor gas in the local Milky Way, or (3) anoutward diffusion of the Sun from a smaller Galactocentric distance.
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| A Minisurvey of Interstellar Titanium from the Southern Hemisphere
We describe the results of a minisurvey of interstellar Ti II and Ca IIabsorption toward 42 early-type stars observed from the southernhemisphere at a spectral resolution of 4.5 km s-1. Results are alsopresented for the Na I ultraviolet line (3302 Angstroms) detected towardnine of these targets. We examine the dependence of the integratedcolumn densities of N(Ti II), N(Ca II), and N(Na I) on distance,reddening, neutral hydrogen column density, and their Galactic elementalabundance. Our findings support the proposition that Ti II and Ca IIabsorption originates in the same regions of the pervasive, warm, andneutral intercloud gas of the interstellar medium. We have observed acorrelation of decreasing Ti and Ca abundance with increasingline-of-sight gas density. The Ti II/Ca II abundance ratio has beenfound to be essentially constant under all the interstellar densityconditions we have sampled. Thus, we conclude that the generalabsorption properties of titanium (and calcium) are similar throughoutthe entire disk of our Galaxy.
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| O VI + Ly beta + C II from Starburst and Poststarburst Galaxies. I. Stellar Library and Evolutionary Synthesis Profiles
Evolutionary synthesis models of a stellar population in thefar-ultraviolet are presented. The spectra include the lines O VI lambdalambda 1032, 1038, Ly beta , and C II lambda lambda 1036, 1037. They arebased on a stellar library built with observations of O and B starscollected with Copernicus and the Hopkins Ultraviolet Telescope (HUT).This library is used as input into an evolutionary synthesis code. Theline profile of O VI + Ly beta + C II is computed for different starformation histories (instantaneous burst and continuous star formation)and different assumptions about the initial mass function (IMF). Themetallicity is near the solar value. O VI lambda lambda 1032, 1038 is avery sensitive indicator of the presence or absence of O stars. O VIdevelops a P Cygni profile when formed in stellar winds of the mostmassive stars. When these stars are absent, no O VI is formed. Incontrast, Ly beta and C II are very sensitive indicators for B stars. Ifthese stars dominate, as is the case in poststarburst galaxies, Ly betaand C II are present as strong absorption features, and they are formedin the photosphere of B stars. An equivalent width of Ly beta + C IIlarger than 1 Angstroms always indicates a population younger than 1Gyr. Because of the universal strength of O VI in O stars, O VI is not agood discriminator between instantaneous versus continuous starformation for ages in the starburst phase, but the absence of O VI andthe presence of stellar Ly beta and C II is a good indicator of a shortburst duration and for the galaxy being in a poststarburst phase.Application of this technique to starburst or poststarburst galaxieswill require careful attention to interstellar absorption.
| ICCD Speckle Observations of Binary Stars.XVIII.An Investigation of Be =
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2112M&db_key=AST
| V 3903 Sagittarii: a massive main-sequence (O7V+O9V) detached eclipsing binary
We present for the first time an analysis based on uvby light curves,Hβ indices and on new spectroscopic data of the massive detacheddouble-lined O-type eclipsing binary V 3903Sgr. The uvby light curvesare analysed with the WINK (initial solutions) and the Wilson-Devinney(WD, final solution) programs. Both codes were used in their extendedversions, with stellar atmospheres and taking into account the geometricdistortions and photometric effects caused by proximity of thecomponents. The spectroscopic CCD observations were analysed with theharmonic ``Wilsing-Russell'' and the ``Lehman-Filhes'' methods. Weconclude that V 3903Sgr is one of the rare O-type detached systems whereboth components are still on the initial phases of the main sequence,with an age of either 1.6x10(6) yrs or 2.5x10(6) yrs (depending on theevolutionary model adopted) at a distance of ~1500pc, the same as forthe Lagoon Nebula (Messier8) complex, of which the system is probably amember. We determine the absolute dimensions: M_A=27.27+/-0.55,R_A=8.088+/-% 0.086, M_B=19.01+/-0.44 and R_B=6.125+/-0.060 (solarunits). There is no evidence of mass transfer and the system isdetached. The orbit is circular, and both components show synchronousrotation, despite their early evolutionary stage. The absolutedimensions determined should be representative for normal single stars.Amongst the massive systems (M>17Msun) with preciseabsolute dimensions (errors <2%), V 3903Sgr is that with the mostmassive primary, with the largest mass difference between thecomponents, and it is the youngest one. Based on data collected with the60$\,$cm and 1.6$\,$m telescopes at the Pico dos Dias Observatory,Na\-tional Laboratory of Astrophysics, LNA-CNPq, Bra\-só\-polis,MG, Brazil and with the Danish 50$\,$cm telescope (SAT) at the EuropeanSouthern Observatory (ESO), La Silla, Chile
| High S/N Echelle spectroscopy in young stellar groups. II. Rotational velocities of early-type stars in SCO OB2.
We investigate the rotational velocities of early-type stars in the ScoOB2 association. We measure v.sin(i) for 156 established and probablemembers of the association. The measurements are performed with threedifferent techniques, which are in increasing order of expectedv.sin(i): 1) converting the widths of spectral lines directly tov.sin(i), 2) comparing artificially broadened spectra of low v.sin(i)stars to the target spectrum, 3) comparing the HeI λ4026 lineprofile to theoretical models. The sample is extended with literaturedata for 47 established members of Sco OB2. Analysis of the v.sin(i)distributions shows that there are no significant differences betweenthe subgroups of Sco OB2. We find that members of the binary populationof Sco OB2 on the whole rotate more slowly than the single stars. Inaddition, we find that the B7-B9 single star members rotatesignificantly faster than their B0-B6 counterparts. We test varioushypotheses for the distribution of v.sin(i) in the association. Theresults show that we cannot clearly exclude any form of randomdistribution of the direction and/or magnitude of the intrinsicrotational velocity vector. We also investigate the effects of rotationon colours in the Walraven photometric system. We show that positions ofB7-B9 single dwarfs above the main sequence are a consequence ofrotation. This establishes the influence of rotation on the Walravencolours, due primarily to surface gravity effects.
| Wind variability of B supergiants. II. The two-component stellar wind of γ Arae.
The stellar wind of the rapidly rotating early-B supergiant, γAra, is studied using time series, high-resolution IUE spectroscopysecured over ~6 days in 1993 March. Results are presented based on ananalysis of several line species, including N V, C IV, Si IV, Si III, CII, and Al III. Comparisons of the time-averaged wind line morphology ofγ Ara to the UV spectra of other OB stars, and to profiles fromspherically symmetric wind models, suggest that the wind of γ Arais equatorially enhanced. Co-existing time variable features areidentified at low-velocity (redward of ~750km/s) and at higher-speedsextending to ~-1500km/s. The observed interface between these structuresis `defined' by the appearance of a discrete absorption component whichis extremely sharp (in velocity space). The central velocity of this`Super DAC' changes only gradually, over several days, between ~-400 and-750km/s in most of the ions. However, its location is shifted redwardby almost 400km/s in Al III and C II, indicating that the physicalstructure giving rise to this feature has a substantial velocity andionization jump. Constraints on the relative ionization properties ofthe wind structures are discussed. The overall wind activity in γAra exhibits a clear ion dependence, such that low-speed features arepromoted in low-ionization species, including Al III, C II, and Si III.We also highlight that - in contrast to most OB stars - there aresubstantial differences in the epoch-to-epoch time-averaged windprofiles of γ Ara and that the high-speed component observedduring our 1993 time series is normally not present. We discuss thepotential roles of the radiative bi-stability mechanism and windcompressed regions due to rapid stellar rotation for providing grosswind distortions in γ Ara.
| The Abundance of Interstellar Oxygen
We present high S/N HST GHRS echelle observations of the weakinterstellar O I] lambda 1356 absorption toward the stars gamma Cas,epsilon Per, delta Ori, epsilon Ori, 15 Mon, tau CMa, and gamma Ara. Incombination with previous GHRS measurements in six other sightlines(zeta Per, xi Per, lambda Ori, iota Ori, kappa Ori, and zeta Oph), thesenew observations yield a mean interstellar gas-phase oxygen abundance(per 10(6) H atoms) of 10(6) O/H = 316 +/- 9. There are no statisticallysignificant variations in the measured O abundances from sightline tosightline and no evidence of density-dependent O depletion from the gasphase. In particular, using the fractional abundance of molecularhydrogen (f(H_2) = 2N(H_2)/[2N(H_2) + N(H I)]) as a barometer ofgas/dust interactions, the mean O abundance in the seven sightlines withlog f(H_2) < -2.0 (10(6) O/H = 318 +/- 13) is essentially identicalto that in the six sightlines with log f(H_2) > -2.0 (10(6) O/H = 314+/- 12). Assuming various dust mixtures of oxides and silicates such asolivine, the abundance of interstellar O tied up in dust grains isunlikely to surpass 10(6) O/H ~ 180 (Cardelli et al. 1996). It isdifficult to further increase this O fraction in grain cores simplybecause the requisite metals are far less abundant than oxygen.Consequently, the GHRS observations imply that the total abundance ofinterstellar oxygen (gas plus grains) in the vicinity of the Sun isabout 2/3 the solar value of 10(6) O/H = 741 +/- 130 (Grevesse &Noels 1993).
| The Pulkovo Spectrophotometric Catalog of Bright Stars in the Range from 320 TO 1080 NM
A spectrophotometric catalog is presented, combining results of numerousobservations made by Pulkovo astronomers at different observing sites.The catalog consists of three parts: the first contains the data for 602stars in the spectral range of 320--735 nm with a resolution of 5 nm,the second one contains 285 stars in the spectral range of 500--1080 nmwith a resolution of 10 nm and the third one contains 278 stars combinedfrom the preceding catalogs in the spectral range of 320--1080 nm with aresolution of 10 nm. The data are presented in absolute energy unitsW/m(2) m, with a step of 2.5 nm and with an accuracy not lower than1.5--2.0%.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Altar |
| Right ascension: | 17h25m23.60s |
| Declination: | -56°22'39.0" |
| Apparent magnitude: | 3.34 |
| Distance: | 348.432 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 3.123 |
| V-T magnitude: | 3.275 |
Catalogs and designations:
|
