Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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.
| 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.
| 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.
| Velocity Distribution of Stars in the Pup-CMa Association
The distribution of proper motions of stars in the Pup-CMa associationis presented. The stars' velocities are approximately parallel to eachother, which indicates that the stars are close together in space. Themutual distribution of stars and molecular clouds in the association isinterpreted as proof that the stars emerged from a single giganticprimordial molecular cloud (or several large clouds), destroyed byradiation and/or stellar wind coming from those stars. It is assumedthat part of that cloud is being dissipated, while part is being brokeninto several small clouds, which we are observing at present.
| 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.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| 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
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| The region of Collinder 121
The distribution of bright B-type stars in a field with a radius of5° centred at the Galactic open cluster Cr 121 is studied utilizingStrömgren and Hβ photometry. All PPM stars earlier thanspectral type A0 are used, revealing a loose nearby structure at adistance of 660-730pc, and a compact more distant group, which appearsto be a genuine cluster: Cr 121. Based on similar coordinates, distancesand positions on the colour-magnitude (CM) and Hertzsprung-Russell (HR)diagrams, 11 photometric cluster members are selected at a mean distanceof 1085(+/-41 standard error) pc. The results are discussed in the lightof both classical and Hipparcos points of view.
| Winds from Hot Stars
This review deals with the winds from "normal" hot stars such asO-stars, B- and A-supergiants, and Central Stars of Planetary Nebulaewith O-type spectra. The advanced diagnostic methods of stellar winds,including an assessment of the accuracy of the determinations of globalstellar wind parameters (terminal velocities, mass-loss rates, windmomenta, and energies), are introduced and scaling relations as afunction of stellar parameters are provided. Observational results areinterpreted in the framework of the stationary, one-dimensional (1-D)theory of line-driven winds. Systematic effects caused by nonhomogeneousstructures, time dependence, and deviations from spherical symmetry arediscussed. The review finishes with a brief description of the role ofstellar winds as extragalactic distance indicators and as tracers of thechemical composition of galaxies at high redshift.
| B Stars as a Diagnostic of Star Formation at Low and High Redshift
We have extended the evolutionary synthesis models by Leitherer et al.by including a new library of B stars generated from the IUEhigh-dispersion spectra archive. We present the library and show how thestellar spectral properties vary according to luminosity classes andspectral types. We have generated synthetic UV spectra for prototypicalyoung stellar populations varying the IMF and the star formation law.Clear signs of age effects are seen in all models. The contribution of Bstars in the UV line spectrum is clearly detected, in particular forgreater ages when O stars have evolved. With the addition of the newlibrary we are able to investigate the fraction of stellar andinterstellar contributions and the variation in the spectral shapes ofintense lines. We have used our models to date the spectrum of the localsuper-star cluster NGC 1705-1. Photospheric lines of C III λ1247,Si III λ1417, and S V λ1502 were used as diagnostics todate the burst of NGC 1705-1 at 10 Myr. Interstellar lines are clearlyseen in the NGC 1705-1 spectrum. Broadening and blueshifts of severalresonance lines are stronger in the galaxy spectrum than in our modelsand are confirmed to be intrinsic of the galaxy. Si II λ1261 andAl II λ1671 were found to be pure interstellar lines with anaverage blueshift of 78 km s-1 owing to a directed outflow ofthe interstellar medium. We have selected the star-forming galaxy1512-cB58 as a first application of the new models to high-z galaxies.This galaxy is at z=2.723, it is gravitationally lensed, and its highsignal-to-noise ratio Keck spectrum shows features typical of localstarburst galaxies, such as NGC 1705-1. Models with continuous starformation were found to be more adequate for 1512-cB58 since there arespectral features typical of a composite stellar population of O and Bstars. A model with Z=0.4 Zsolar and an IMF with α=2.8reproduces the stellar features of the 1512-cB58 spectrum.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| An analysis of Almagest magnitudes for the study of stellar evolution.
Not Available
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| New OB-Association in Pup - CMA
Not Available
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| 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
| The wind momentum-luminosity relationship of galactic A- and B-supergiants
The Balmer lines of four A Ia-supergiants (spectral type A0 to A3) andfourteen B Ia and Ib-supergiants (spectral type B0 to B3) in the solarneighbourhood are analyzed by means of NLTE unified model atmospheres todetermine the properties of their stellar winds, in particular theirwind momenta. As in previous work for O-stars (Puls et al. \cite{pul96})a tight relationship between stellar wind momentum and luminosity(``WLR'') is found. However, the WLR varies as function of spectraltype. Wind momenta are strongest for O-supergiants, then decrease fromearly B (B0 and B1) to mid B (B1.5 to B3) spectral types and becomestronger again for A-supergiants. The slope of the WLR appears to besteeper for A- and mid B-supergiants than for O-supergiants. Thespectral type dependence is interpreted as an effect of ionizationchanging the effective number and the line strength distributionfunction of spectral lines absorbing photon momentum around the stellarflux maximum. This interpretation needs to be confirmed by theoreticalcalculations for radiation driven winds. The ``Pistol-Star'' in theGalactic Centre, an extreme mid B-hypergiant recently identified as oneof the most luminous stars (Figer et al. \cite{fig99}) is found tocoincide with the extrapolation of the mid B-supergiant WLR towardshigher luminosities. However, the wind momentum of the Luminous BlueVariable P Cygni, a mid B-supergiant with extremely strong mass-loss, is1.2 dex higher than the WLR of the ``normal'' supergiants. Thissignificant difference is explained in terms of the well-known stellarwind bi-stability of supergiants very close to the Eddinton-limit inthis particular range of effective temperatures. A-supergiants in M31observed with HIRES at the Keck telescope have wind momenta compatiblewith their galactic counterparts. The potential of the WLR as a new,independent extragalactic distance indicator is discussed. It isconcluded that with ten to twenty objects, photometry with HST andmedium resolution spectroscopy with 8m-telescopes from the grounddistance moduli can be obtained with an accuracy of about 0fm1 out tothe Virgo and Fornax clusters of galaxies.
| Galactic B-supergiants: A non-LTE model atmosphere analysis to estimate atmospheric parameters and chemical compositions
A non-LTE model atmosphere analysis of moderate resolution (R ~ 5 000)spectra of 46 Galactic B-type supergiants is presented. Standardtechniques are adopted, viz. plane-parallel geometry and radiative andhydrostatic equilibrium. Spectroscopic atmospheric parameters (T_eff,log g & v_turb) and chemical abundances (He, C, N, O, Mg & Si)are estimated, both as a test of the validity of such an approach and inan attempt to provide consistent results for supergiants covering asignificant range of spectral types. The values of the estimatedatmospheric parameters and their dependence on the physics adopted inthe model atmospheres calculations are discussed. The absolute metalabundances are compared to those of main sequence B-type stars and, ingeneral, their chemical compositions appear to be similar. Theabundances for He, C, N & O are considered in some detail and arediscussed in the context of possible evolutionary histories for thisstellar sample. Specifically, it is found that the supergiant sample canbe subdivided into a number of evolutionarily distinct groups. The lowermass objects are predominantly chemically near-normal i.e. theirphotospheres show little or no evidence for chemical processing, whereasthe higher mass supergiants have CNO ratios which are indicative of CNand possibly NO-cycle burning. An attempt is made to quantify thedifference in nitrogen and carbon abundances between the high and lowmass targets but this is hampered by theoretical uncertainties. Thepossibilities that the most highly processed supergiants may have eitherlarger rotational velocities or have undergone mass transfer within abinary system are discussed.
| Study of an unbiased sample of B stars observed with Hipparcos: the discovery of a large amount of new slowly pulsating B stars
We present a classification of 267 new variable B-type stars discoveredby Hipparcos. We have used two different classification schemes and theyboth result in only a few new beta Cephei stars, a huge number of newslowly pulsating B stars, quite some supergiants with alpha Cyg-typevariations and variable CP stars, and further some new periodic Be starsand eclipsing binaries. Our results clearly point out the biased naturetowards short-period variables of earlier, ground-based surveys ofvariable stars. The position of the new beta Cephei stars and slowlypulsating B stars in the HR diagram is determined by means of Genevaphotometry and is confronted with the most recent calculations of theinstability strips for both groups of variables. We find that the newbeta Cephei stars are situated in the blue part of the instability stripand that the new slowly pulsating B stars almost fully cover thetheoretical instability domain determined for such stars. Thesupergiants with alpha Cyg-type variations are situated between theinstability strips of the beta Cephei and the slowly pulsating B starson the one hand and previously known supergiants that exhibitmicrovariations on the other hand. This suggests some connection betweenthe variability caused by the kappa mechanism acting in a zone ofpartially ionised metals and the unknown cause of the variations insupergiants.
| UV Spectral Classification of O and B Stars in the Small Magellanic =
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1951N&db_key=AST
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| 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.
| Accurate Two-dimensional Classification of Stellar Spectra with Artificial Neural Networks
We present a solution to the long-standing problem of automaticallyclassifying stellar spectra of all temperature and luminosity classeswith the accuracy shown by expert human classifiers. We use the 15Angstroms resolution near-infrared spectral classification systemdescribed by Torres-Dodgen & Weaver in 1993. Using the spectrum withno manual intervention except wavelength registration, artificial neuralnetworks (ANNs) can classify these spectra with Morgan-Keenan types withan accuracy comparable to that obtained by human experts using 2Angstroms resolution blue spectra, which is about 0.5 types (subclasses)in temperature and about 0.25 classes in luminosity. Accuratetemperature classification requires a hierarchy of ANNs, whileluminosity classification is most successful with a single ANN. Wepropose an architecture for a fully automatic classification system.
| Systematic Errors in the FK5 Catalog as Derived from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..850S&db_key=AST
| Metallicities of 4 blue supergiants near the Galactic centre.
High resolution optical spectra of four blue supergiants (HD 148422B0.5Ib; HD 178487 B0.5Ib; HD 179407 B1Ib, HD 163522 B1Ib) which liewithin 4.5kpc of the Galactic centre are presented. Careful differentialLTE model atmosphere analyses are used to quantify the differences inphotospheric metal abundances between these stars and MK spectralstandards in the solar neighborhood. A detailed non-LTE model atmosphereanalysis of one star (HD 163522) confirms that the LTE differentialabundances should be reliable, provided we use a comparison star withsimilar atmospheric parameters. One star (HD 148422; R_g_=4.5kpc) has achemical composition similar to normal nearby B-type stars while HD178487 (R_g_=4.1kpc) shows a mild but consistent pattern of a metalenhanced atmosphere (with elements up by 0.1-0.3dex). The other twostars HD 163522 (R_g_=4.1kpc) and HD 179407 (R_g_=3.5kpc) havesignificantly higher metal compositions than their standard comparisonstars with abundances enhanced by 0.30-0.40dex and 0.20-0.5dexrespectively. All of the stars lie outside the Galactic plane (withdistances in the range 0.5
| Revised spectral types for 64 B-supergiants in the Small Magellanic Cloud: metallicity effects.
The problem of the classification of metal poor stars, such as occur inthe Small Magellanic Cloud (SMC), is discussed with reference to theapplicability of the MK system in such an environment. An alternativemethod is presented here and applied to B-type supergiants in the SMC. Alocal reference system is first devised and then a transformation to MKspectral types is determined by comparing the trends of metal linestrengths in these two systems. For the determination of the luminosityclass, we emphasize the need to use the hydrogen Balmer line strengthsindependently of metal line-strength considerations. This method is usedto determine new spectral types for 64 supergiants in the SMC, 75% ofthe sample requiring classifications different from previous findings.These new types result in much improved line strength - spectral typecorrelations for He, C, N, O, Mg and Si. Corresponding changes in thedistribution of these stars in the Hertzsprung-Russell diagram of theSMC reveal more clearly than before the existence of a ridge which maybe the SMC analogue of a similar feature found for the LMC byFitzpatrick & Garmany (1990ApJ...363..119F). The group of veryluminous supergiants lying above this ridge includes the LBV AV415(R40), a property which this object has in common with LBVs in the LargeMagellanic Cloud. Also, for the first time, clear examples of BN/BCsupergiants are found in the SMC.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Велики пас |
| Ректацензија: | 07h03m01.50s |
| Deклинација: | -23°50'00.0" |
| Apparent магнитуда: | 3.02 |
| Даљина: | 787.402 parsecs |
| Proper motion RA: | -3 |
| Proper motion Dec: | 3.9 |
| B-T magnitude: | 2.906 |
| V-T magnitude: | 2.999 |
Каталог и designations:
|
INHABITED SKY
