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B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the 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/431/773
| Interstellar Matter near the Pleiades. VI. Evidence for an Interstellar Three-Body Encounter
This paper seeks a comprehensive interpretation of new data on Na Iabsorption toward stars in and near the Pleiades, together with existingvisible and infrared data on the distribution of dust and with radiodata on H I and CO in the cluster vicinity. The use of dust and gasmorphology to constrain tangential motions in connection with themeasured radial velocities yields estimates for the space motion of gasnear the Pleiades. Much of the kinematic complexity in the interstellarabsorption toward the Pleiades, including the presence of stronglyblueshifted components that arise in shocked gas, finds explanation inthe interaction between the cluster and foreground gas withVr(LSR)~7 km s-1 associated with the Taurus dustclouds. Taurus gas, however, cannot readily account for an absorptioncomponent having Vr(LSR)~10 km s-1 with a wide,but not continuous distribution and 21 cm emission from gas in thecluster having Vr(LSR)~0 km s-1 associated witheast-west dust filaments. Successive hypotheses for the origin of theseadditional features include Taurus gas at a higher velocity than thepervasive foreground component, additional gas at a radial velocityintermediate between that of the Taurus component and the cluster, and acloud having Vr(LSR)~10 km s-1 approaching thePleiades from the west. A satisfactory account of the full complexity ofthe interstellar medium near the Pleiades requires the last feature andthe Taurus gas, both interacting with the Pleiades and also with eachother.
| Kinematical Structure of the Local Interstellar Medium: The Galactic Anticenter Hemisphere
A survey of interstellar Na I D1 and D2 absorption features in thespectra of 104 early-type stars in the second and third Galacticquadrants reveals the large-scale kinematics of the interstellar gaswithin the Galactic anticenter hemisphere at distances from the Sunbetween ~70 and ~250 pc. Employing a technique that uses both the radialvelocities and column densities of the Na I absorptions produced by theintervening gas we have identified the velocity vectors and determinedthe spatial distribution of eight interstellar clouds in the volumeexplored. The average internal H I+H2 densities of the cloudsrange between 0.03 and 1.7 cm-3, and their masses between 80and 104 Msolar, although uncertainties in thesizes of the clouds, their possible extension beyond the regionexplored, and the presence of denser gas embedded in the larger cloudsimply that these will tend to be lower limits. We have clearlyidentified clumps of denser gas immersed in the low-density gas in oneof the clouds; these clumps show internal H I+H2 densities oforder 50 cm-3. Although we are not able to detect anyinterstellar Na I within 70 pc, the sizes of some of the clouds implythat their near edges are within that range of distances from the Sun.With respect to the local standard of rest the clouds move withvelocities between 19 and 54 km s-1. Their velocity vectorsdo not support the view of a local interstellar medium uniquelydominated by expansion from centers in the Scorpio-Centaurus OBassociation; our results suggest that this expansion is present in theGalactic center hemisphere but in the Galactic anticenter hemisphere isrestricted to the immediate neighborhood of the Sun.
| The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry
We have used wide-field ultraviolet, optical, and far-infraredphotometric images of Pleiades reflection nebulosity to analyze dustproperties and the three-dimensional nebular geometry. Scattered lightdata were taken from 1650 and 2200 Å Wide-Field Imaging SurveyPolarimeter images and a large 4400 Å mosaic of Burrell SchmidtCCD frames. Dust thermal emission maps were extracted from IRAS data.The scattering geometry analysis is complicated by the blending of lightfrom many stars and the likely presence of more than one scatteringlayer. Despite these complications, we conclude that most of thescattered light comes from dust in front of the stars in at least twoscattering layers, one far in front and extensive, the other nearer thestars and confined to areas of heavy nebulosity. The first layer can beapproximated as an optically thin, foreground slab whose line-of-sightseparation from the stars averages ~0.7 pc. The second layer is alsooptically thin in most locations and may lie at less than half theseparation of the first layer, perhaps with some material among orbehind the stars. The association of nebulosities peripheral to the maincondensation around the brightest stars is not clear. Models withstandard grain properties cannot account for the faintness of thescattered UV light relative to the optical. Some combination ofsignificant changes in grain model albedo and phase function asymmetryvalues is required. Our best-performing model has a UV albedo of0.22+/-0.07 and a scattering asymmetry of 0.74+/-0.06. Hypotheticaloptically thick dust clumps missed by interstellar sight linemeasurements have little effect on the nebular colors but might shiftthe interpretation of our derived scattering properties from individualgrains to the bulk medium.
| The Pleiades Reflection Nebula. I. Ultraviolet, Optical, and Far-Infrared Imaging Photometry
We present new wide-field optical and ultraviolet images of the Pleiadesreflection nebula that allow a more thorough evaluation of the dustscattering than any prior data set. Vacuum-UV images were taken at 1650and 2200 Å during the first flight of the Wide-Field ImagingSurvey Polarimeter (WISP), a sounding rocket-borne telescope. WISPcaptured the brighter parts of the nebula at both wavelengths, with 3σ sensitivities of 22.5 and 23.4 UV mag arcsec-2,respectively. The 5.0d×1.7d WISP field was also mapped at 4400Å with a mosaic of 40 Burrell Schmidt CCD frames using a broadbandBJ filter. The Schmidt mosaic shows extensive and intricatenebulosity down to a 5 σ sensitivity limit of 27.6 B magarcsec-2, including features undetected by photographicsurveys. We explore the intensity and color behavior of the nebula inour UV and optical images and far-infrared IRAS data. We find that thephotometric structure near bright stars is more complex than previousstudies have implied, but general trends are still apparent. The colorgradients around the stars are caused by phase function effects ratherthan internal reddening. The greater concentration of scattered lightversus thermal emission indicates that most of the observed scatteringis from foreground dust. A somewhat greater concentration of UV versusoptical light suggests grain scattering is more forward-directed atshorter wavelengths. The UV nebula is much fainter than expected fromthe stellar photometry and interstellar reddening. Explaining this UVfaintness requires either more reddening than is measured or significantalterations to current dust property estimates.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Metallicities of the SPB stars from the IUE ultraviolet spectra
We derived the stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddenings for 20 SPB and34 reference stars observed during the IUE satellite mission. Theparameters were derived by means of an algorithmic procedure of fittingtheoretical flux distributions to the low-resolution IUE spectra andoptical spectrophotometric observations. Since the metallicity [m/H] hasa special importance for pulsating B type stars, we focused ourattention on that parameter. We found that the mean value of themetallicity of the considered SPB and reference stars amounts to [m/H] ~-0.20. The results only slightly depend on the reduction procedure usedfor the IUE images (NEWSIPS and INES). The metal abundances obtained inthis paper are in accordance with the average value of -0.2 dex forstars in the solar neighborhood recently reported by otherinvestigators.Tables 3-7 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/404/689
| Quantitative Stellar Spectral Classification. II. Early Type Stars
The method developed by Stock & Stock (1999) for stars of spectraltypes A to K to derive absolute magnitudes and intrinsic colors from theequivalent widths of absorption lines in stellar spectra is extended toB-type stars. Spectra of this type of stars for which the Hipparcoscatalogue gives parallaxes with an error of less than 20% were observedwith the CIDA one-meter reflector equipped with a Richardsonspectrograph with a Thompson 576×384 CCD detector. The dispersionis 1.753 Å/pixel using a 600 lines/mm grating in the first order.In order to cover the spectral range 3850 Å to 5750 Å thegrating had to be used in two different positions, with an overlap inthe region from 4800 Å to 4900 Å . A total of 116 stars wasobserved, but not all with both grating positions. A total of 12measurable absorption lines were identified in the spectra and theirequivalent widths were measured. These were related to the absolutemagnitudes derived from the Hipparcos catalogue and to the intrinsiccolors (deduced from the MK spectral types) using linear and secondorder polynomials and two or three lines as independent variables. Thebest solutions were obtained with polynomials of three lines,reproducing the absolute magnitudes with an average residual of about0.40 magnitudes and the intrinsic colors with an average residual of0.016 magnitudes.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Computing the Parallax of the Pleiades from the Hipparcos Intermediate Astrometry Data: An Alternative Approach
The inconsistency between the mean parallax of the Pleidaes open clusterfrom the Hipparcos catalog and that obtained from the stellar evolutiontheory and photometric measurements is probed by recomputing theHipparcos data in a different way that reduces the propagation of thealong-scan attitude errors. This is achieved by coupling observations ofstars made nearly simultaneously in the two separate fields of view ofthe telescope. A direct calculation of astrometric quantities of 54Pleiades members by the new method, based on the Intermediate AstrometryData, provides a correction of -0.71+/-0.14 mas to the weighted meanparallax of the cluster. The mean corrected parallax of the Pleiades is7.75+/-0.20 mas.
| Pre-main-sequence stars in the young open cluster NGC 1893. I. A spectroscopic search for candidates in the area photometrically surveyed
We present low-resolution spectroscopy of the majority of the stars inthe field of NGC 1893 which were classified asnon-members based on the photometry of Marco et al. (2001). We separatefield stars from pre-main-sequence members based on a combination ofspectral type and photometric indices which allows us to determine thereddening to the objects. For candidates to pre-main-sequence objects,we present intermediate-resolution spectra. We confirm 3 F-typeemission-line objects as massive T Tauri stars in NGC1893 and find two good candidates for being absorption-lineHerbig A stars. A B5 star is a very good candidate for anabsorption-line Herbig B star. We also investigate the nature ofemission-line B-type stars close to the cluster centre. Availableevidence suggests that both classical Be stars and Herbig Be stars arepresent in NGC 1893. Based on observations obtainedat the Asiago Observatory (Italy), the Isaac Newton Telescope (La Palma,Spain) and Observatoire de Haute Provence (CNRS, France).
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| 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
| Interstellar Matter Near the Pleiades. V. Observations of NA I toward 36 Stars
This paper reports high-resolution, moderate to high signal-to-noiseratio observations of 23 certain Pleiades members, four possiblemembers, and nine nonmembers in the Na I D lines, as well asobservations of 12 of the stars in the Na I ultraviolet doublet. Inspite of the relative proximity of the stars to the sun (even most ofthe nonmembers lie within 200 pc), the line profiles exhibit remarkablecomplexity, with up to five absorption components and equally remarkablestar-to-star variation. The velocity range, 2-20 km s-1,conforms well to the range expected for gas deflected by the passage ofthe cluster. The paper includes a careful discussion of uncertainties inthe data, the most important conclusions of which are that the velocityscatter is consistent with that expected from random errors in thewavelength calibration and that systematic errors probably are <~0.1km s-1. Appendices detail the choice of stellar data and theprocedure adopted for removing telluric absorption lines. Analysisfollows in a separate paper.
| Barnard's Merope Nebula Revisited: New Observational Results
IC 349 is a small, fan-shaped reflection nebula located only 30" from 23Tau; its nucleus is, by a factor 15, the brightest area of the Pleiadesnebulosity. We propose that IC 349 is a fragment of the Taurus-Aurigamolecular cloud that has been encountered by the Pleiades in thatcluster's southward motion and is being illuminated and shaped by theradiation field of 23 Tau. New Hubble Space Telescope multicolor imageryand the structure, colors, and surface brightness of IC 349 arediscussed in terms of that hypothesis. What is known of the propermotion of the nebula, what can be inferred of the properties of thenebula from its color, and what is expected from radiation pressuretheory appear to be compatible with this cloudlet-encounter hypothesis.
| The IMF of open star clusters with Tycho-2
We studied the fields of nine nearby open star clusters based on theTycho-2 catalogue. We determined membership probabilities for the starsin the cluster fields from the stellar proper motions and used theTycho-2 photometry to compute the initial mass function (IMF) of theclusters from the main sequence turn-off point down to approx. 1 M_sun.We found IMF slopes ranging from Gamma =-0.69 down to Gamma =-2.27 (whenthe Salpeter \cite{salpeter} value would be Gamma =-1.35). We alsostudied the membership of individual stars of special astrophysicalinterest. In some cases previous results had to be revised. As aby-product, we investigated some general properties of the Tycho-2catalogue; we confirmed that the Tycho-2 proper motions show onlymarginal deviations from the Hipparcos catalogue. On the other hand, insome regions the completeness of the catalogue seems to decrease atmagnitudes brighter than claimed by Høg et al. (\cite{tycho2}).Based on observations of the ESA Hipparcos satellite.
| Internal kinematics and binarity of X-ray stars in the Pleiades open cluster
The classical convergent point analysis is implemented for the Pleiadesstars with proper motions in the Tycho-2 Catalogue and X-ray fluxesmeasured by the ROSAT satellite. It is demonstrated that, with thestandard astrometric errors as given in Tycho-2, strong X-ray sources inthe cluster (log L_X > 29.1, where L_X is in erg s-1)exhibit a velocity dispersion in one component of only 0.20 kms-1, while the distributions of velocity components ofmoderate (log LX < 29.1) sources and stars not detected byROSAT at all are consistent with a velocity dispersion of 0.64 kms-1. The difference is statistically significant at the levelof 1.6sigma , or 0.95 confidence limit. This result is a clue to thekinematics/X-ray luminosity segregation, similar to that previouslydiscovered in the Hyades open cluster. It is discussed that thesegregation may be caused by a wide spread of ages of the member stars.The occurrence of high X-ray luminosities is found to correlate verywell with visual binarity and multiplicity (separations > 10 AU).
| 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
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is 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/367/297
| Another Search for Maia Variable Stars
We have used the Hipparcos epoch photometry database, andautocorrelation analysis, to search for the elusive Maiavariables-short-period B7-A3 near-main-sequence pulsating variablestars. Of several hundred stars considered, and several dozen starsstudied in detail, only a handful are possible variables: three arepossible shallow eclipsing variables; three have possible periods in therange 0.25-0.5 day, but their amplitudes are so low that they areprobably nonvariable. The most promising are HD 29573, with a period of1.6 days (but possibly a rotating variable), and γ CrB, with aperiod of 0.9 day-a period also found spectroscopically by Lehmann andcoworkers. Sirius shows variations which are probably instrumental. Twopreviously suspected Maia stars-Maia and γ UMi-are photometricallyconstant. The Maia variables-if they exist-are very rare and veryelusive.
| Search for X-ray flares in the Pleiades using SoHO LASCO C3 images.
Not Available
| On the Variability of Late B III-V Stars
We investigate the Hipparcos Satellite photometry of luminosity classIII-V B6-B9 stars. Most are relatively non-variable. Candidates forwhich further study is desirable are identified.
| ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.
| Barnard's Merope Nebula (IC 349): an Interstellar Interloper
Barnard's Merope Nebula (IC 349) is the optically brightest portion ofthe diffuse nebulosity that envelops the Pleaides but is notmorphologically similar to those nebulae. Knowledge of its true spacemotion can help clarify whether the object has a kinematic associationand possibly a common origin with the Pleiades. Here we report a meanradial velocity result obtained in 1996 from spectra where v_hel=-44.4km s^-1 and sigma_v=5.42 km s^-1 (N=5). The radial velocity result ispresented along with recent values for the object's proper motion,yielding its space motion vector. Galactic space velocity components (U,V, W)=(50.6+/-5.3, -10.3+/-6.7, 11.3+/-6.4) km s^-1, referred to theLSR, were calculated for the object. In addition, the region wasobserved in the near-infrared to determine if a protostellar object ispresent within the dusty envelope of the nebula; to an equivalentluminosity upper limit of L=0.23+/-0.05 L_solar, none was observed.These results suggest that IC 349 is kinematically unrelated to thePleiades and that it does not harbor a protostellar object in its dustyinterior.
| CCD spectra of MK standards and a preliminary extension of the MK classification to the yellow-red region.
Not Available
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Deep ROSAT HRI observations of the Pleiades
In a deep X-ray survey of the Pleiades open cluster, we use the ROSATHigh Resolution Imager to explore a region of the cluster formerlysurveyed with the PSPC. These new observations substantially improveupon both the sensitivity and the spatial resolution for this region ofthe Pleiades, allowing us to detect 18 cluster members not detectedbefore and 16 members not included in the catalogs used in previoussurveys. The high sensitivity of the present observations permits us toobtain more stringent upper limits for 72 additional members and alsoprovides sufficient numbers of stars to enable us to explore thedependence of L_x on stellar rotation for the slow rotators of thePleiades. Using the new high sensitivity X-ray observations and therecent rotational measurements we discuss the activity-rotationrelationship in the Pleiades solar type stars. We also present newphotometric observations of optical counterparts of a number of X-raysources detected in previous surveys but not yet identified. Table~2 isavailable in electronic form at CDS via ftp 130.79.128.5
| Les premieres retombees astrophysiques d'Hipparcos.
Not Available
| The Pleiades and alpha Persei Clusters
The upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%.
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Osservazione e dati astrometrici
| Costellazione: | Toro |
| Ascensione retta: | 03h45m12.50s |
| Declinazione: | +24°28'02.0" |
| Magnitudine apparente: | 4.3 |
| Distanza: | 114.286 parsec |
| Moto proprio RA: | 20.1 |
| Moto proprio Dec: | -44.8 |
| B-T magnitude: | 4.158 |
| V-T magnitude: | 4.272 |
Cataloghi e designazioni:
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