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Ýlgili Makaleler
Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| 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.
| Highly ionized gas in the local ISM: Some like it hot?
We present HST-STIS medium-resolution spectra (R 6.5 kms-1) of the ultraviolet interstellar absorption linesobserved towards 4 early-type stars located within the localinterstellar medium (ISM), with sight-line distances <186 pc in thegeneral direction of the Loop I superbubble (l = 330°, b =+18°). These data have been supplemented with high resolution (R 3 km s-1) visible absorption observations of the NaID-lines towards these 4 stars. Our main discovery is the detection ofhighly ionized absorption components of CIV, SiIV and NV towards the twomost distant targets, HD 127381 and HD 142256. These lines-of-sight areknown to cross both the near and far neutral interface boundaries to theLoop I cavity, in addition to intersecting the fragmented shell ofneutral and partially ionized gas that defines the boundary to the LocalBubble. However, the presently measured narrow line profile-widths andtheir measured absorption intensities are found to be incompatible withtheoretical models that predict high ion absorption due to the presenceof evaporating cloud conduction interfaces. We conclude that theformation of high ions in the local ISM is highly dependent on thelocation of the absorbing gas clouds with respect to nearby sources ofboth hot X-ray emitting gas and/or photo-ionization. Our observationshave also revealed at least 6 gas clouds with distances ranging from 5pc to 150 pc along these sight-lines. We have detected a cloud ofneutral and partially ionized gas with a velocity of -15 kms-1 and a hydrogen column density of log N(HI +HII) 19.3 cm-2 that is thought to define the boundary to the LocalBubble cavity at a distance of ~90 pc in this galactic direction. Thefar neutral boundary to the Loop I superbubble cavity is also detectedat a distance of 150-180 pc and is composed of two cold clouds moving atvelocities close to Vhelio 0 km s-1possessing a combined hydrogen column density of log N(HI + HII)≫19.5 cm-2. In contrast, we have also detected three lowdensity, warm and partially ionized diffuse clouds with averagevelocities of ~-10, -23 and -32 km s-1, that are all locatedwithin a distance of ~150 pc. The cloud component at V -23 kms-1 may be associated with the very local "G-cloud" at adistance of <5 pc, but we also provide evidence for its placement ata greater distance. The measured velocities of the majority of the gasclouds we have detected along all 4 sight-lines are consistent with aninflow of gas into the LB cavity from the direction of the Loop Isuperbubble. This gas is flowing through a region of fragmentation at adistance of ~90 pc that represents the interaction region between theLoop I and Local Bubble cavities.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Kinematics of Hipparcos Visual Binaries. II. Stars with Ground-Based Orbital Solutions
This paper continues kinematical investigations of the Hipparcos visualbinaries with known orbits. A sample, consisting of 804 binary systemswith orbital elements determined from ground-based observations, isselected. The mean relative error of their parallaxes is about 12% andthe mean relative error of proper motions is about 4%. However, even 41%of the sample stars lack radial velocity measurements. The computedGalactic velocity components and other kinematical parameters are usedto divide the stars with known radial velocities into kinematical agegroups. The majority (92%) of binaries from the sample are thin diskstars, 7.6% have thick disk kinematics and only two binaries have halokinematics. Among them, the long-period variable Mira Ceti has a verydiscordant {Hipparcos} and ground-based parallax values. From the wholesample, 60 stars are ascribed to the thick disk and halo population.There is an urgent need to increase the number of the identified halobinaries with known orbits and substantially improve the situation withradial velocity data for stars with known orbits.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| A study of interstellar Nai D absorption lines towards the Lupus molecular clouds
Intermediate-resolution (60000<=R<=120000) observations ofinterstellar Nai lines towards 29 stars in the general direction of theLupus molecular clouds (330°<~l<~350°0°<~b<~25°) are presented. Previously published spectratowards an additional seven stars are also included. Based on theHipparcos distances to these stars, and the minimum distance at whichstrong interstellar Nai lines appear in the spectra, I obtain a distanceof ~150+/-10pc to the Lupus molecular complex. While in agreement with anumber of other independent estimates, this result is at odds with thevalue of 100pc recently obtained by Knude & Høg from aHipparcos-based study of interstellar extinction. A possible explanationfor this discrepancy is discussed, and it is concluded that the value of150+/-10pc obtained here is to be preferred. In addition, theseobservations have some other implications for the structure of theinterstellar medium in this direction, and these are briefly considered.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| 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.
| A near IR adaptive optics search for faint companions to early-type multiple stars
We report on a high dynamical range (>10(m) ) and high angularresolution (down to 0.2arcsec ) search for low-mass components ofearly-type primaries in 7 intermediate-age (5 Myr) multiple systems withthe ESO 3.6 m telescope and the adaptive optics system ADONIS. Theimages were obtained in the J and SK bands with and without acoronographic mask of 2arcsec diameter. The census is nearly complete inthe angular separation range 1arcsec - 6arcsec , corresponding to linearseparations of 200-1200 AU, which have remained unstudied so far due tothe intrinsic brightness of the massive primaries. The best detectionlimits are around K=15(m) at 1arcsec and K=19(m) at 5arcsec separationfrom the primary. In 13 fields of 12.5arcsec x 12.5arcsec 6 new faintstars were detected. Their magnitudes and colours indicate that 2 ofthem can be physical low-mass components. The bright (K=11.2, J-K=1.2)companion to HD 108250 C at 2.17arcsec has an IR excess and deservesfurther study. Several fields were explored without a mask; somewhatsurprisingly, no new components at sub-arcsecond separations down to0.2arcsec were detected. The first order stability analysis indicatesthat the studied multiple systems are hierarchical at spatial scales of40-1000 AU. The data are briefly discussed in the light of the formationand evolution of multiple systems with massive primaries. Based onobservations collected at the ESO La Silla Observatory (program58.D-0455).
| A new determination of the local distribution of interstellar NaI
We present galactic maps of the distribution of neutral sodiumabsorption column density using the newly determined Hipparcos satellitedistances for a total of 290 stars lying within 250pc of the Sun. Thesemaps now replace those originally presented in Welsh {et al./} (1994).The Local Bubble is now seen to be a neutral gas-free cavity(nH < 0.01 cm(-3) ) with a radius of at least 70pc in mostdirections, more than 50% larger in volume than previously determinedfrom NaI absorption data. Additionally, the well-known low-densityinterstellar tunnel in the direction of the star beta CMa is almosttwice as wide as previously thought, and the existence of a similarlytenuous interstellar feature has been identified in the galacticdirection of Lupus-Norma extending to at least 170pc. Finally, we notethe absence of dense neutral gas out to a distance of at least 100pc inthe direction of galactic longitude {lII} = 135(deg) .
| ROSAT PSPC Observations of 27 Near--Main-Sequence B Stars
In this paper, we report on ROSAT Position Sensitive ProportionalCounter (PSPC) observations of 27 near--main-sequence B stars made withunprecedented sensitivity. Contrary to the results of previous surveys,it is found that 75% of the sample stars are X-ray sources, albeit mostat modest levels. The X-ray luminosities of the program stars range from5.6 x 1027 up to 2.2 x 1032 ergs s-1. We find that LX/LBol decreasesabruptly beyond about B0 and stabilizes at LX/LBol ~ 10-8.5 by about B2,with seven nondetections at B2 and later. For the B0 and B1 stars, ourmodeling suggests that wind attenuation of the X-ray photons issignificant, so that the emitted X-ray luminosity, corrected for thisattenuation, actually exceeds 10-7LBol in some cases. Presumably, thissituation is even more severe for O stars; thus, the well-known LX/LBol~ 10-7 law simply may be an artifact of the neglect of wind attenuation.The ROSAT PSPC observations of most of the B stars are very soft, withthe notable exception of tau Sco (B0 V). The wind emission measurefilling factors that we find for the very early B stars are rather large(roughly 0.1--1). This could be brought into line with theoreticalcalculations of the line-force instability, wind-shock mechanism if themass-loss rates of these stars are a few times higher than theorycurrently predicts. However, the X-rays from stars later than B2 requirefilling factors greater than unity and thus cannot be produced by anyradiation-driven wind-shock mechanism because there is simply not enoughwind material to produce the observed X-rays. It is possible that mid-to late-B stars represent some kind of transition to, or hybrid of, windand coronal X-ray mechanisms.
| Kinematical Structure of the Local Interstellar Medium: The Galactic Center Hemisphere
A survey of interstellar Na I D1 and D2 absorption features found in thespectra of 31 early-type stars in the first and fourth Galacticquadrants reveals a new picture of the large-scale kinematics ofinterstellar gas in the vicinity of the Sun. While absorption featuresfound in the spectra of stars in the fourth Galactic quadrant show thegas expanding from stars in the Scorpio-Centaurus OB association, starsin the first Galactic quadrant, in the range of distances covered bythis survey (74--312 pc from the Sun), show interstellar gas fallingtoward the Galactic plane with a bulk motion whose component parallel tothat plane is directed toward the Scorpio-Centaurus association. Thispicture differs completely from generally accepted models in which thenearby interstellar gas flows away from the Scorpio-Centaurusassociation toward the Galactic anticenter. Evidence for the presence ofan expanding shell-like feature that is reaching the solar system ispresented.
| MSC - a catalogue of physical multiple stars
The MSC catalogue contains data on 612 physical multiple stars ofmultiplicity 3 to 7 which are hierarchical with few exceptions. Orbitalperiods, angular separations and mass ratios are estimated for eachsub-system. Orbital elements are given when available. The catalogue canbe accessed through CDS (Strasbourg). Half of the systems are within 100pc from the Sun. The comparison of the periods of close and widesub-systems reveals that there is no preferred period ratio and allpossible combinations of periods are found. The distribution of thelogarithms of short periods is bimodal, probably due to observationalselection. In 82\% of triple stars the close sub-system is related tothe primary of a wide pair. However, the analysis of mass ratiodistribution gives some support to the idea that component masses areindependently selected from the Salpeter mass function. Orbits of wideand close sub-systems are not always coplanar, although thecorresponding orbital angular momentum vectors do show a weak tendencyof alignment. Some observational programs based on the MSC aresuggested. Tables 2 and 3 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
| Micrometer measurements of southern double stars made at The Observatory of Llano del Hato at Merida (Venezuela)
We present 93 micrometer measurements of 46 double stars observed withthe 65 cm refractor at the Llano del Hato Observatory (Merida,Venezuela).
| 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.
| 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%.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The determination of T_eff_ of B, A and F main sequence stars from the continuum between 3200 A and 3600 A.
A method of determination of the effective temperature of B, A and Fmain sequence stars is proposed, using the slope of the continuumbetween 3200A and 3600A. The effective temperature calibration is basedon a sample of stars with energy distributions known from UV to the red.We have determined the Balmer jump and the effective temperatures for235 main sequence stars. The temperatures found have been compared withthose derived by Underhill et al. (1979), Kontizas & Theodossiou(1980), Theodossiou (1985), Morossi & Malagnini (1985). Thecomparison showed good agreement for most of the stars in common. On theother hand, the temperatures derived from the reddening-free colourfactor QUV, from the colour index (m1965-V) and from (B-V), given inGulati et al. (1989), are systematically lower than our temperatures,however the differences are within one-sigma error.
| Decoupled line driven outflow around B and Be stars.
The semi-analytic model of Bjorkman & Cassinelli for the equatorial`discs' around Be stars is re-examined and shown to be capable ofproducing higher equator-to-pole density contrasts when the effects ofthe dynamical decoupling of the stellar radiation field from theoutflowing gas are incorporated. The enhancement, as measured by thenormal component of the momentum fed into the disc, is in the region ofa factor of 2. Inclusion of the effect also lowers the threshold stellarrotation rate for the formation of a wind-compressed disc fromv_rot_/v_crit_=~0.4 to 0.25. Two physical mechanisms that may give riseto this decoupling, which are most effective in B star winds, arepresented and their relative merits compared. This comparison isperformed with reference to available UV and X-ray observations.Ion-stripping (Springmann & Pauldrach 1992) could be the cause ofthe low observed maximum outflow velocities, but it cannotsimultaneously explain the ubiquitous high-temperature (>10^6^K)X-ray emission. Shock disruption of the wind ionization balance (Castor1987) may also terminate the outward radiative acceleration in the windsof B stars. This effect suggests a simple method for setting an upperlimit to the X-ray emission from weak shock-disrupted winds whichcompares reasonably favourably with X-ray luminosities derived fromROSAT data.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| Physical characteristics of close binary system components
An approximate approach for evaluating the mass of invisible satellitesof close binary systems with the mass-function f(M) much less than 1 issuggested. A possibility of using it is shown for 62 close binarysystems.
| ICCD speckle observations of binary stars. VIII - Measurements during 1989-1991 from the Cerro Tololo 4 M telescope
One-thousand eighty-eight observations of 947 binary star systems,observed by means of speckle interferometry with the 4 m telescope onCerro Tololo, are presented. These measurements, made during the period1989-1991, comprise the second installment of results stemming from theexpansion of our speckle program to the southern hemisphere.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
| Night-sky brightness from the visibility of stars near the horizon
Hundreds of naked-eye observations of bright stars near the southernhorizon have been made on clear moonless nights over the interval1977-91. They reveal a consistency in their visibility from one seasonor year to another, but a deterioration over the entire time intervalreveals a possible increase in light pollution and possibly airpollution as well. The method is constrained to the sky near thenorthern and southern horizons where stars remain at near-constantaltitude over a suitable time interval, but the repeatability assuresthat it is sound. The limiting magnitude is given as a function ofzenith distance, and color and seasonal effects do not appear toinfluence the results.
| High resolution observations of interstellar NA I and CA II towards the Scorpio-Centaurus association
High-resolution observations (3.6 km/s FWHM) of interstellar Na I and CaII toward 28 stars in the direction of the Sco-Cen association arepresented. The observed velocity structure, velocity dispersions, columndensities and Na I/Ca II ratios are discussed. Of the 58 identifiedvelocity components (toward 23 stars), 46 (or 79 percent) were found tohave negative LSR velocities. This is in agreement with previousresults, and indicates a general outflow from the Sco-Cen association.By fitting an expanding shell model to the observed velocities anexpansion velocity of 7 km/s was obtained. If the further assumption ismade that only one component per star arises in such a shell, thebest-fitting expansion velocity was found to be 9 km/s. In the lattercase, the remaining components were variously identified (depending ontheir velocities and Na I/Ca II ratios).
| ICCD speckle observations of binary stars. V - Measurements during 1988-1989 from the Kitt Peak and the Cerro Tololo 4 M telescopes
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99..965M&db_key=AST
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| Takýmyýldýz: | Kurt |
| Sað Açýklýk: | 15h35m08.50s |
| Yükselim: | -41°10'01.0" |
| Görünürdeki Parlaklýk: | 2.78 |
| Uzaklýk: | 173.913 parsek |
| özdevim Sað Açýklýk: | -28.2 |
| özdevim Yükselim: | -42 |
| B-T magnitude: | 2.755 |
| V-T magnitude: | 2.954 |
Kataloglar ve belirtme:
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