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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.
| 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.
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| 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.
| Physical Parameters of Southern B- and Be-Type Stars
In this paper we present new results on stellar fundamental parametersfor early B and Be field stars observed in the southern hemisphere:effective temperature, superficial gravity, and projected stellarrotation velocity. The estimation of their projected rotation velocitiesis made by two successive methods. We first obtain an initial valuebased on Fourier transforms of the He I λ4471 line for 34 B andBe field stars with magnitudes in the range0.5<=mv<=10, followed by a more accurate fittingprocedure of observed lines with non-LTE model line profiles. Thisprocedure yields stellar rotation velocity estimates that are inagreement with those of the literature. We derive also Teffand logg values by fitting equivalent widths and profiles of NLTE modelspectra to the observed ones. Finally, we give estimates of stellarages, masses, and bolometric luminosities derived from interpolations inthe evolutionary tracks calculated by Schaller.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| 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
| Estimation of the mass loss, opening angle and mass of Be circumstellar disks from Brmathsf γ continuum emission and interferometric measurements
Using the SIMECA code developed by Stee & Araùjo(\cite{stee1}); Stee et al. (\cite{stee2}) for Be stars we obtain acorrelation between the mass loss rates {dot M} and the Brgammacontinuum luminosity as a function of the opening angle of the disk. Weshow that this correlation is similar to those obtained by Scuderi etal. (\cite{scuderi}) for O-B supergiants. We found that the wind densityat the base of the photosphere, from a sample of 8 Be stars, liesbetween 10-13 and 10-12 g cm-3. We alsopresent a relationship between the mass of the circumstellar disk andthe 2.16 mu m flux. Finally we emphasize how interferometricmeasurements can help to estimate the wind density and we present asample of 16 Be stars with predicted visibilities that can be observedwith the VLTI.
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
| 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
| The Hard X-Ray Luminosity of OB Star Populations: Implications for The Contribution of Star Formation to the Cosmic X-Ray Background
We present an empirical analysis of the integrated X-ray luminosityarising from populations of OB stars. In particular, we utilize resultsfrom the All-Sky Monitor on RXTE, along with archival data from previousmissions, to assess the mean integrated output of X-rays in the 2-10 keVband from accreting early-type binaries within 3 kpc of the Sun. Using arecent OB star census of the solar neighborhood, we then calculate thespecific X-ray luminosity per O star from accretion-powered systems. Wealso assess the contribution to the total X-ray luminosity of an OBpopulation from associated T Tauri stars, stellar winds, and supernovae.We repeat this exercise for the major Local Group galaxies, concludingthat the total X-ray luminosity per O star spans a broad range from 2 to20×1034 ergs s-1. Contrary to previousresults, we do not find a consistent trend with metallicity; in fact,the specific luminosities for M31 and the SMC are equal, despite havingmetallicities that differ by an order of magnitude. In light of theseresults, we assess the fraction of the observed 2-10 keV emission fromstarburst galaxies that arises directly from their OB star populations,concluding that, while binaries can explain most of the hard X-rayemission in many local starbursts, a significant additional component orcomponents must be present in some systems. A discussion of the natureof this additional emission, along with its implications for thecontribution of starbursts to the cosmic X-ray background, concludes ourreport.
| High and intermediate-resolution spectroscopy of Be stars 4481 lines
We present an atlas of Hγ , He i lambda 4471 and Mg ii lambda 4481line profiles obtained in a 10 year observation period of 116 Be stars,which enabled many of them to be observed at quite different emissionepochs. From the best fit of the observed He i lambda 4471 line profileswith non-LTE, uniform (Teff,log g) and full limb-darkenedmodel line profiles, we determined the V sin i of the program stars. Toaccount, to some degree, for the line formation peculiarities related tothe rapid rotation-induced non-uniform distributions of temperature andgravity on the stellar surface, the fit was achieved by considering(Teff,log g) as free parameters. This method produced V sin iestimations that correlate with the rotational velocities determined bySlettebak (1982) within a dispersion sigma <= 30 km s-1and without any systematic deviation. They can be considered as given inthe new Slettebak's et al. (1975) system. Only 13 program stars havediscrepant V sin i values. In some objects, this discrepancy could beattributed to binary effects. Using the newly determined V sin iparameters, we found that the ratio of true rotational velocitiesV/Vc of the program Be stars has a very low dispersion aroundthe mean value. Assuming then that all the stars are rigid rotators withthe same ratio V(/lineω)/Vc, we looked for the value of/line ω that better represents the distribution of V sini/Vc for randomly oriented rotational axes. We obtained/lineω = 0.795. This value enabled us to determine the probableinclination angle of the stellar rotation axis of the program stars. Inthe observed line profiles of Hγ , He i lambda 4471, Mg ii lambda4481 and Fe ii lambda 4351 we measured several parameters related to theabsorption and/or emission components, such as: equivalent width,residual emission and/or absorption intensity, FWHM, emission peakseparations, etc. The parameters related to the Hγ line emissionprofiles were used to investigate the structure of the nearbyenvironment of the central star. From the characteristics of thecorrelations between these quantities and the inferred inclinationangle, we concluded that in most of cases the Hγ line emissionforming regions may not be strongly flattened. Using a simplerepresentation of the radiation flux emitted by the star+envelopesystem, we derived first order estimates of physical parameterscharacterizing the Hγ line emission formation region. Thus, weobtained that the total extent of the Hγ region is Rf=~ 2.5 +/- 1.0 R* and that the density distribution in theselayers can be mimicked with a power law rho ~ R-alpha , wherealpha =2.5+2.2-0.6. The same approach enabled usto estimate the optical depth of the Hγ line emission formationregion. From its dependence with the aspect angle, we concluded thatthese regions are caracterized by a modest flattening and that the rho(equator)/rho (pole) density contrast of the circumstellar envelope nearthe star should be two orders of magnitude lower than predicted bymodels based on a priori disc-shaped circumstellar envelopes. We foundthat the separation between the emission peaks, Deltap, andthe full width at half maximum, Delta 1/2, of the Hγline emission are not only sensitive to kinematic effects, but to lineoptical depth as well. This finding agrees with previous theoreticalpredictions and confirms that Huang's (1972) relation overestimates theextent of the Hγ line emission formation region. Data obtained atCASLEO operated under agreement between the CONICET and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina, at ESOLa Silla, Chile and at OHP, France.}\fnmsep\thanks{Tables 2 to 7 andFigs. 1 and 2 are only available in full in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/861}} \subtitle{Anatlas of Hγ , He {\fontsize {10pt}{12pt}\selectfont I} 4471 and Mg{\fontsize {10pt}{12pt}\selectfont II
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| 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
| A catalogue of high-mass X-ray binaries
We present a catalogue of high-mass X-ray binaries. The catalogue is anupdated version of the catalogue of van Paradijs (1995). This newcatalogue contains 130 sources, 61 new high-mass X-ray binaries inaddition to the 69 sources listed in van Paradijs' catalogue. Most ofthe new sources are identified to be Be/X-ray binaries. Some sources,however, are only tentatively identified as high-mass X-ray binaries onthe basis of a transient character and/or a hard X-ray spectrum. Furtheridentification in other wavelength bands is needed to finally determinethe features of these sources. The aim of this catalogue is to providethe reader with some basic information on the X-ray sources and theircounterparts in other wavelength ranges (UV, optical, IR, radio). Incases where there is some doubt about the high-mass nature of the X-raybinary this is mentioned. Some doubtful cases have not been included inthe catalogue although they had been suggested by some authors to behigh-mass X-ray binaries, for example GRS 1915+105 and 1WGAJ1958.2+3232. The catalogue and references are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://vizier.u-strasbg.fr/cgi-bin/qcat?J/A+AS/147/25
| On the origin of the difference between the runaway velocities of the OB-supergiant X-ray binaries and the Be/X-ray binaries
The recent finding by Chevalier & Ilovaisky (1998) from Hipparcosobservations that OB-supergiant X-ray binaries have relatively largerunaway velocities (mean peculiar tangential velocity avvt = 42 +/- 14\kms), whereas Be/X-ray binaries have low runaway velocities (avvt = 15+/- 6 \kms), provides confirmation of the current models for theformation of these two types of systems. These predict a difference inrunaway velocity of this order of magnitude. This difference basicallyresults from the variation of the fractional helium core mass as afunction of stellar mass, in combination with the conservation oforbital angular momentum during the mass transfer phase that precededthe formation of the compact object in the system. This combinationresults into: (i) Systematically narrower pre-supernova orbits in theOB-supergiant systems than in the Be-systems, and (ii) A largerfractional amount of mass ejected in the supernovae in high-mass systemsrelative to systems of lower mass. Regardless of possible kickvelocities imparted to neutron stars at birth, this combination leads toa considerable difference in average runaway velocity between these twogroups. If one includes the possibility for non-conservative masstransfer the predicted difference between the runaway velocity of thetwo groups becomes even more pronounced. The observed low runawayvelocities of the Be/X-ray binaries confirm that in most cases not morethan 1 to 2 \msun\ was ejected in the supernovae that produced theirneutron stars. This, in combinatioThe values given here are not identical (though similar) to those listedin Chevalier & Ilovaisky (1998). The corrections we applied areoutlined below.
| 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
| 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.
| On the nature of Be/X-ray binaries
It has been suggested that most Be/X-ray binaries are low X-rayluminosity nearby objects, containing white dwarfs (Chevalier &Ilovaisky 1998). We show that existing evidence indicates that all knownBe/X-ray binaries are relatively bright X-ray sources containing neutronstars and that the spectral distribution of this group differsconsiderably from that of isolated Be stars. We suggest that thedifferent X-ray properties of the systems can be explained by the sizesof the orbits of the neutron stars. Systems with close orbits are brighttransients which show no quiescent emission as a consequence ofcentrifugal inhibition of accretion. Systems with wide orbits arepersistent sources and display no large outbursts. Systems withintermediate orbits present a mixture of both behaviours.
| HIPPARCOS results on massive X-ray binaries
We present results on parallaxes, magnitudes and proper motions for 17optically identified massive X-ray binaries (MXRB) which were observedduring the Hipparcos astrometry mission. This sample includes thesources detected in the medium-energy range (2-10 keV) having opticalcounterparts brighter than V=12. We compare the Hipparcos results withground-based optical data and derive probable values for absolutemagnitudes and peculiar tangential velocities (vt). The 4 OBsupergiant systems in our sample are high-velocity objects (the averagevalue of vt is greater than 60 km/s), while the 13 Be systemshave low velocities ( = 11.3 +/- 6.7 km/s),suggesting a different formation mechanism for the two subgroups. Theunusual X-ray/radio source LSI+61(deg) 303 lies much closer thanpreviously believed and its low optical luminosity (MV =+2.2) and blue intrinsic colors now suggest it may be a peculiar type oflow-mass X-ray binary.
| ICCD Speckle Observations of Binary Stars.XVIII.An Investigation of Be =
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2112M&db_key=AST
| 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.
| On the confinement of one-armed oscillations in discs of Be stars.
We discuss the effects due to the rotational deformation and theoptically-thin line force on the confinement of one-armed oscillationsin the inner part of Be-star discs. The period of these oscillations isidentified with the observed V/R variations in Balmer emission lines. Wetake into account the effect of rotation by including the quadrupolecontribution to the potential around the rotationally-distorted centralstar. For the radiative force due to an ensemble of optically thinlines, we adopt the parametric form proposed by Chen & Marlborough(1994ApJ...427.1005C). The disc is assumed to be isothermal. Based onthese assumptions, we examine the linear, one-armed eigenmodes confinedto the inner part of the disc. Our study strongly suggests that themechanism that causes the confinement of one-armed oscillations inearly-type Be stars is different from that in late-type Be stars. Inlate-type Be stars, the confinement occurs because of the deviation fromthe point-mass potential around the rotationally deformed star. On thispoint, we confirm the conclusions obtained by Papaloizou et al.(1992A&A...265L..45P). In early-type Be stars, however, it is theweak-line force that mainly contributes to the confinement. Therotational effect plays a much smaller role for these stars. The periodof the eigenmode confined to the disc depends sensitively on the effectby rotation or radiation. This sensitiveness, together with the range ofthe period of observed V/R variations, places rather narrow constraintson the parameters characterizing these effects. We compare our resultswith observed V/R properties, for which a list of 53 stars has beencompiled.
| 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.
| Atlas of high-resolution emission and shell lines in Be stars. Line profiles and short-term variability.
We present an atlas of high-S/N, high-resolution ({DELTA}v=6km/s) dataof Be star emission and shell profiles. We have collected profiles ofHα and of Fe II, mostly of the λ5317 transition. Theselines have been selected to provide measures for the overall emissionstrength and for the velocity field in these disks. We have collecteddata for 77 southern and equatorial programme stars, covering the period1982-1993. This is the most comprehensive overview of profile shapes inBe disks. We propose a three-dimensional scheme in which most observedprofiles can be classified. The parameters are i) inclination, ii)optical depth, and iii) the pattern of the velocity field. A search forshort-term variability (timescales between five days and a few minutes)in six stars ended with negative result. Shortest observed timescale forvariability is a few days for well-developed disks in binary systems (HR1910, HR 2142).
| On the structure of Be star disks.
We investigate the geometrical structure of the emitting part ofcircumstellar envelopes around Be stars from an empirical point of view.We use new high-resolution, high-S/N spectroscopic data of the FeIIλ5317 and some other faint FeII emission lines in 27 Be starsshowing symmetrical emission lines (class 1). We find a clearcorrelation between its total width (measuring the maximum velocities ofcircumstellar matter) and the stellar rotational velocity. Thiscorrelation means that a typical Be envelope (or, more precisely, thatpart of it which is visible in optical emission lines) is anaxisymmetric, rotationally supported disk. For empirical investigationof the vertical structure, we use the occurrence of shell lines. Wedefine, as shell criterion based on FeII lines, a Be shell star as onewith FeII central intensity F_cd_/F_*_(FeII)<1. Using this forcalibrating an appropriate parameter for the much more frequentlyobserved Hα line, we find that shell stars are those withF_p_/F_cd_(Hα)>=1.5 where F_p_ is the mean peak intensity atHα. In a sample of 114 programme stars, we find a shell starfraction of 22.8%. This number is readily transformed into a halfopening angle of Be star disks, φ=13deg. We furthermore show thatBe disks must be thin at the inner edge, and may become fairly thick atthe outer rim. This, together with the small value of φ, isevidence for a conical or concave shape, the latter typical of ahydrostatically balanced disk. Finally we provide evidence that thefamous "shell-Be" phase transitions can naturally occur in such disks asa geometrical effect if they are seen under inclination i=~70deg and iftheir outer radius is variable with time.
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