תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The Magnetic Field Strength in the Wall of the Local Bubble toward l, b ~ 300°, 0°
We present polarization measurements in the region around l, b =300°, 0°, toward stars with well-determined distances, primarilyin the range 40-200 pc. We argue that these observations tracepolarization caused by dust in the wall of the Local Bubble (LB). Usinga Chandrasekhar-Fermi analysis, we derive a plane-of-the-sky magneticfield strength, in the wall of the LB in this direction, of⊥>=8+5-3 μG, equivalent to a magneticpressure of PB/k~1.8×104 K cm-3.This pressure equals, to within the uncertainties, that in the bubbleinterior, as derived from EUVE observations, assuming collisionalionization equilibrium. Such a magnetic field could help us explain theobserved low O VI column density associated with the LB wall.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type starsmembers of the subgroups Upper Scorpius, Upper Centaurus Lupus and LowerCentaurus Crux of the nearby Sco-Cen OB association. The radial velocitymeasurements were obtained by means of high-resolution echelle spectravia analysis of individual lines. The internal accuracy obtained in themeasurements is estimated to be typically 2-3 km s-1, butdepends on the projected rotational velocity of the target. Radialvelocity measurements taken for 2-3 epochs for the targets HD 120307, HD142990 and HD 139365 are variable and confirm that they arespectroscopic binaries, as previously identified in the literature.Spectral lines from two stellar components are resolved in the observedspectra of target stars HD 133242, HD 133955 and HD 143018, identifyingthem as spectroscopic binaries.
| A Hot Envelope around the Southern Coalsack: X-Ray and Far-Ultraviolet Observations
We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-rayobservations toward the Southern Coalsack. An almost complete X-ray halocan be seen around the cloud in the 0.75 and 1.5 keV images, and most ofthe observed stars show O VI absorption. Both the cloud and the starshave highly accurate distance determinations, allowing us to reliablyplace the stars and the cloud relative to each other. Using thesedistance determinations, we find no O VI-bearing gas in the foregroundof the Coalsack, while for stars in the background of the cloud, O VIabsorption is the norm. The column density of O VI correlates with the0.75 and 1.5 keV intensities. These results suggest that theX-ray-emitting hot plasma is associated with the dense cloud. We proposethat the heating of the Coalsack envelope is due to the hot gas in theinterior of the Upper Cen-Lup superbubble. The Coalsack interactionregion provides a nearby example of the hot-cold gas interfaces thoughtto be responsible for the O VI absorptions seen on many sight linesthroughout the Galaxy.
| 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
| Ice in the Southern Coalsack
To better understand the conditions under which ice mantles form ongrains in molecular clouds, three globules in the Southern Coalsack havebeen searched for the presence of H2 O ice. Given the totallack of star formation in the Coalsack, it is an ideal site for studyingunprocessed icy molecular mantles. In our sample of eight field starslying behind the Coalsack we detect strong H2 O iceabsorption in the lines of sight to two stars and possible weakabsorption in four others. We estimate H2 O ice columndensities or upper limits for these lines of sight. Compared to darkclouds such as Taurus, the Coalsack H2 O ice column densitiesare lower than expected given the quiescent nature of the Coalsackregion. It is possible that the chemical evolution of the Coalsack maysimply be at too early a stage for significant ice mantles to appear onthe grains, except perhaps in the densest parts of some of the globules.Alternatively, the presence or absence of ice absorption may be relatedto the distribution of dust along each line of sight, specifically, therelative contributions of dense globules and a more extended diffusecomponent. For example, our observations are consistent with an icethreshold extinction similar to that observed in the Taurus dark cloudif extinction amounting to A V ~5 towards Globules 2 and 3arises in the extended component. Globule 1 appears to have no extendedcomponent.
| The mass ratio distribution of B-type visual binaries in the Sco OB2 association
A sample of 115 B-type stars in the Sco OB2 association is examined forexistence of visual companions in the J and K_s bands, using the ADONISnear-infrared adaptive optics system and coronograph. Practically allthe components in the separation range 0farcs3 -6farcs4 (45-900 AU) andmagnitudes down to K = 16 were detected. The K and J - K photometry ofthe primaries and differential photometry and astrometry of the 96secondaries are presented. Ten secondaries are new physical components,as inferred from the photometric and statistical criteria, while therest of the newly detected objects are faint background stars. After asmall correction for detection incompleteness and a conversion of thefluxes into masses, an unbiased distribution of the components massratio q was derived. The power law f(q)~ q-0.5 fits theobservations well, whereas a q-1.8 distribution, whichcorresponds to a random pairing of stars, is rejected. The companionstar fraction is 0.20+/-0.04 per decade of separation which iscomparable to the highest measured binary fraction among low-mass PMSstars and ~ 1.6 times higher than the binary fraction of low-mass dwarfsin the solar neighborhood and in open clusters in the same separationrange. Based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO programme 65.H-0179). Tables 1, 3 andthe full version of Table 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/cgi-bin/qcat?J/A+A/382/92
| 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 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.
| Variable Interstellar Absorption toward the Halo Star HD 219188: Implications for Small-Scale Interstellar Structure
Within the last 10 yr, strong, narrow Na I absorption has appeared atvsolar~-38 km s-1 toward the halo star HD 219188;that absorption has continued to strengthen, by a factor of 2-3, overthe past 3 yr. The line of sight appears to be moving into/through arelatively cold, quiescent intermediate-velocity (IV) cloud, because ofthe 13 mas yr-1 proper motion of HD 219188; the variations inNa I probe length scales of 2-38 AU yr-1. UV spectra obtainedwith the Hubble Space Telescope GHRS in 1994-1995 suggestN(Htot)~4.8×1017 cm-2, ``halocloud'' depletions, nH~25 cm-3, andne~0.85-6.2 cm-3 (if T~100 K) for the portion ofthe IV cloud sampled at that time. The relatively high fractionalionization, ne/nH>~0.034, implies that hydrogenmust be partially ionized. The N(Na I)/N(Htot) ratio is veryhigh; in this case, the variations in Na I do not imply large localpressures or densities. Based in part on observations with the NASA/ESAHubble Space Telescope, obtained from the data archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| Boron Abundances in Early B Stars: Results from the B III Resonance Line in IUE Data
We have used archival International Ultraviolet Explorer high-dispersionSWP spectra to study the B III resonance line at 2065.8 Å in 44early B stars. We find a median boron abundance about half that of solarsystem meteoritic material, consistent with the values found fromprevious studies of boron in early B stars. About one-third of the starsstudied appear to have boron abundances that are a factor of 4 or morelower than this median. Many of these are stars with enhanced nitrogenabundances, confirming the belief that deep envelope mixing can occur inmain-sequence B stars. A few stars with low boron abundances have normalN/C ratios. It is unclear whether all of these can be explained as starswhere mixing depleted the boron but did not go deep enough to bringCN-processed material to the surface or if some stars were actuallyformed with an unusually low boron abundance.
| 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
| Determination of the effective gravity and equatorial rotational velocity by means of the continuum and hydrogen lines
Two simple approaches for determining the effective gravity of singlerotating stars are suggested. In the first case, this gravity can befound by means of the characteristics from a study of a continuum only.In the second case, both characteristics of continuum and those of linespectrum must be used at the same time. Investigations shows that withincalculation errors the two approaches give identical values of theeffective gravity. The knowledge of this characteristic allows toestimate the equatorial rotational velocity and the inclination angle ofrotation axis of a rotator to the line-of-sight, respectively.
| A NA I Absorption Map of the Small-Scale Structure in the Interstellar Gas toward M15
Using the DensePak fiber-optic array on the KPNO WIYN telescope, we haveobtained high signal-to-noise ratio echelle spectra of the Na I Dwavelength region toward the central 27^''x43^'' of the globular clusterM15 at a spatial resolution of 4". The spectra exhibit significantinterstellar Na I absorption at LSR velocities of 3 km s^-1 (localinterstellar medium [LISM] component) and 68 km s^-1(intermediate-velocity cloud [IVC] component). Both components varyappreciably in strength on these scales. The derived Na I columndensities differ by a factor of 4 across the LISM absorption map and bya factor of 16 across the IVC map. Assuming distances of 500 and 1500 pcfor the LISM and IV clouds, respectively, these maps show evidence ofsignificant ISM structure down to the minimum scales of 2000 and 6000 AUprobed in these absorbers. The smallest scale N(Na I) variationsobserved in the M15 LISM and IVC maps are typically comparable to orhigher than the values found at similar scales in previous studies ofinterstellar Na I structure toward binary stars. The physicalimplications of the small and larger scale Na I features observed in theM15 maps are discussed in terms of variations in the H I column densityas well as in the Na ionization equilibrium.
| 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.
| Mapping the contours of the Local bubble: preliminary results
We present preliminary results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,taking advantage of Hipparcos stellar distances. Equivalent widths ofthe NaI D-line doublet at 5890 Å are presented for thelines-of-sight towards some 143 new target stars lying within 300 pc ofthe Sun. Using these data which were obtained at the Observatoire deHaute Provence, together with previously published NaI absorptionmeasurements towards a further 313 nearby targets, we present absorptionmaps of the distribution of neutral gas in the local interstellar mediumas viewed from 3 different galactic projections. In particular, thesemaps reveal the Local Bubble region as a low neutral densityinterstellar cavity in the galactic plane with radii between 65-250 pcthat is surrounded by a (dense) neutral gas boundary (or ``wall''). Wehave compared our iso-column contours with the contours derived bySnowden et al. (\cite{snowden98}) using ROSAT soft X-ray emission data.Consistency in the global dimensions derived for both sets of contoursis found for the case of a million degree hot LB plasma of emissivity0.0023 cm(-6) pc with an electron density of 0.005 cm(-2) . We havedetected only one relatively dense accumulation of cold, neutral gaswithin 60 pc of the Sun that surrounds the star delta Cyg, and note thatthe nearest molecular cloud complex of MBM 12 probably resides at thevery edge of the Local Bubble at a distance of ~ 90 pc. Our observationsmay also explain the very different physical properties of the columnsof interstellar gas in the line-of-sight to the two hot stars epsilonCMa and beta CMa as being due to their locations with respect to theBubble contours. Finally, in the meridian plane the LB cavity is foundto be elongated perpendicularly to the Gould's Belt plane, possiblybeing ``squeezed'' by the expanding shells of the Sco-Cen andPerseus-Taurus OB associations. Tables 1 and 2 are also available inelectronic form at the CDS (Strasbourg) via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| On the normal spectral energy distribution of stars: Spectral types O9-B5
The normal energy distributions for fifteen spectral subtypes from O9 toB5 for luminosity classes V, IV, and III are derived. Threephotometrically uniform catalogs served as the source of thespectrophotometric data used. Synthetic color indices for all spectraltypes are calculated using the energy distribution curves obtained.Comparison of these indices with the expected normal color indicessuggests that the energy distributions derived are reliable.
| The Physical Characteristics of the Small-Scale Interstellar Structure toward MU Crucis
We present Hubble Space Telescope Goddard High Resolution Spectrographechelle observations of multiple interstellar lines of C i, Mg i, Cr ii,and Zn ii toward both stars in the mu Crucis binary system. Despitelarge differences in the profiles of the neutral species, no significantvariations between the stars are seen in the Cr ii and Zn ii lineprofiles. In particular, the Zn ii absorption observed at -8.6 km s^-1toward mu Cru is constant despite greatly enhanced columns of theneutral species at this velocity toward mu^1 Cru. An analysis of thefine-structure excitation of C i in this cloud implies that the densityis n_H<250 cm^-3. From the lack of variation in the (optical) Na i D2line profiles toward mu^1 and mu^2 Cru in spectra taken 21 months apart,we can place a lower limit to the size of the structures of ~10 AU.These results are discussed in the context of recent radio and opticalstudies of apparently pervasive high-density small-scale interstellarstructure. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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%.
| Small-Scale Interstellar Medium Structure: The Remarkable Sight Line toward MU Crucis
We present very high resolution ( Delta v ~ 0.4 km s-1) observations ofthe interstellar Na I D and Ca II K lines toward the binary mu Cru,which consists of two B stars separated by 38."8 at a distance of 170pc. These observations reveal line strength variations in four velocitycomponents, indicative of interstellar medium structure on scales lessthan the projected binary separation of 6600 AU (0.03 pc). Thecomponents exhibiting the greatest Na I column density variations havethe narrowest line widths and the largest N(Na I)/N(Ca II) ratios. Themost visually striking of these variations involves a Na I componentseen at a heliocentric velocity of -8.6 km s-1 toward mu 1 Cru but notat all toward mu 2 Cru. This component has an appreciable column density[N(Na I) = 7 x 1010 cm-2], and it clearly exhibits hyperfine splittingwith a Na I line width (b = 0.4 km s-1) indicative of a cloudtemperature below 220 K. It is likely that the mu Cru variations aresampling a widespread pattern of small-scale structure in the colddiffuse interstellar gas that could easily have been missed in previousobservations.
| 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.
| Small-Scale ISM Structure: The Remarkable Sightline Toward MU CRU
Observations of interstellar absorption lines toward both members ofresolvable binary star systems can provide a sensitive probe ofsmall-scale structure in the diffuse ISM\@. We have recently obtainedvery high resolution (Delta v ~ 0.3 km s(-1) ) observations of theinterstellar Na I D lines toward several binaries with the Ultra-HighResolution Facility (UHRF) on the 3.9 m Anglo-Australian Telescope. Inthe case of mu Cru, which consists of two B stars separated by 38farcs8at a distance of 170 pc, these observations reveal line strengthvariations in at least four velocity components indicative of ISMstructure on scales less than the projected binary separation of 6600 AU(0.03 pc). The most striking of these variations involves a Na Icomponent seen at a heliocentric velocity of -9 km s(-1) toward mu (1)Cru but not at all toward mu (2) Cru. This component has an appreciablecolumn density (N(Na I) = 7 x 10(10) cm(-2) ) and it clearly exhibitshyperfine splitting with a Na I line width (FWHM ~ 0.7 km s(-1) )indicative of a cloud temperature below 200 K\@. We have also obtainedUHRF spectra of the interstellar Ca II K line toward mu Cru which reveala N(Na I)/N(Ca II) ratio (~7) for the -9 km s(-1) component that issuggestive of some dust depletion but is much lower than that of typicaldiffuse clouds. The other obvious profile differences in this sightlineinclude a pair of cold components near +4 km s(-1) that are much morepronounced toward mu (2) Cru than mu (1) Cru. Given the remarkablevariations in multiple velocity components toward mu Cru, it should notbe too surprising that our high resolution observations of Na I towardother binaries reveal that such small-scale structure in the cold ISM iscommon. We will discuss the implications of these results for a bettermicrophysical understanding of diffuse interstellar clouds.
| 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.
| 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.
| Chemical abundances in early B-type stars. 5: Metal abundances and LTE/NLTE comparison
Chemical abundances of neon, magnesium, aluminum, sulfur, and iron arederived for a sample of 21 unevolved B-stars in the local field andnearby associations. While aluminum, sulfur, and iron are underabundantin nearly all stars, near solar abundances are found for magnesium andneon. In agreement with earlier results for carbon, nitrogen, oxygen,and silicon (Kilian 1992), the present results show no correlation withsurface gravities or evolutionary states, which indicates that the metalabundances reflect the original composition of the interstellar medium.The results are supplemented by a comparison of local thermodynamicequilibrium (LTE) and non-LTE (NLTE) abundances for C, N, O, Si, Mg, andAl. In most cases the differences amount to +/- (0.1-0.2) dex, whichslightly exceeds the estimated accuracy of the NLTE abundancedetermination. However, a clear temperature gradient is evident for mostelements, which indicates systematic LTE abundance errors with a maximumamplitude of 0.4 dex between 21 000 K and 31 000 K.
| The distribution of interstellar dust in the solar neighborhood
We surveyed the IRAS data base at the positions of the 1808 O6-B9.5stars in The Bright Star Catalog for extended objects with excessemission at 60 microns, indicating the presence of interstellar dust atthe location of the star. Within 400 pc the filling factor of theinterstellar medium, for dust clouds with a density greater than 0.5/cucm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the densitydistribution function appears to follow a power law index - 1.25. Whenthe dust clouds are mapped onto the galactic plane, the sun appears tobe located in a low-density region of the interstellar medium of widthabout 60 pc extending at least 500 pc in the direction of longitudes 80deg - 260 deg, a feature we call the 'local trough'.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | צלב דרומי |
| התרוממות ימנית: | 12h54m35.60s |
| סירוב: | -57°10'40.0" |
| גודל גלוי: | 4.03 |
| מרחק: | 115.741 פארסק |
| תנועה נכונה: | -29.1 |
| תנועה נכונה: | -12 |
| B-T magnitude: | 3.744 |
| V-T magnitude: | 3.938 |
קטלוגים וכינוים:
|
