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A revisit to agglomerates of early-type Hipparcos stars
% We study the spatial structure and sub-structure of regions rich in{Hipparcos} stars with blue B_T-V_T colours. These regions, whichcomprise large stellar complexes, OB associations, and young openclusters, are tracers of on-going star formation in the Galaxy. TheDBSCAN (Density-Based Spatial Clustering of Applications with Noise)data clustering algorithm is used to look for spatial overdensities ofearly-type stars. Once an overdensity, ``agglomerate'', is identified,we carry out a data and bibliographic compilation of their star membercandidates. The actual membership in agglomerate of each early-type staris studied based on its heliocentric distance, proper motion, andprevious spectro-photometric information. We identify 35 agglomerates ofearly-type {Hipparcos} stars. Most of them are associated to previouslyknown clusters and OB associations. The previously unknown P Puppisagglomerate is subject of a dedicated study with Virtual Observatorytools. It is actually a new, nearby, young open cluster (d ˜ 470pc, age ˜ 20 Ma) with a clear radial density gradient. We list PPuppis and other six agglomerates (including NGC 2451 A, vdBH 23, andTrumpler 10) as new sites for substellar searches because of theiryouth, closeness, and spatial density. We investigate in detail thesub-structure in the Orion, CMa-Pup and Pup-Vel OB complexes(``super-agglomerates''). We confirm or discover some stellaroverdensities in the Orion complex, like the 25 Ori group, the Horseheadregion (including the σ Orionis cluster), and the η Orionisagglomerate. Finally, we derive accurate parallactic distances to thePleiades, NGC 2451 A, and IC 2391, describe several field early-typestars at d < 200 pc, and discuss the incompleteness of our search.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| Shocked Clouds in the Vela Supernova Remnant
Unusually strong high-excitation C I has been detected in 11 lines ofsight through the Vela supernova remnant (SNR) by means of UV absorptionline studies of IUE data. Most of these lines of sight lie near thewestern edge of the bright X-ray region of the SNR in a spatiallydistinct band approximately 1° by 4° oriented approximatelynorth-south. The high-excitation C I (denoted C I* and C I**) isinterpreted as evidence of a complex of shocked dense clouds interactingwith the SNR, because of the high pressures indicated in this region. Tofurther analyze the properties of this region of enhanced C I* and CI**, we present new HIRES-processed IRAS data of the entire Vela SNR. Atemperature map calculated from the HIRES IRAS data, based on atwo-component dust model, reveals the signature of hot dust at severallocations in the SNR. The hot dust is anticorrelated spatially withX-ray emission, as would be expected for a dusty medium interacting witha shock wave. The regions of hot dust are strongly correlated withoptical filaments, supporting a scenario of dense clouds interior to theSNR that have been shocked and are now cooling behind the supernovablast wave. With few exceptions, the lines of sight to the stronghigh-excitation C I pass through regions of hot dust and opticalfilaments. Possible mechanisms for the production of the unexpectedlylarge columns of high-excitation C I are discussed. Dense clouds on theback western hemisphere of the remnant may explain the relatively lowX-ray emission in the western portion of the Vela SNR due to the slowerforward shock velocity in regions where the shock has encountered thedense clouds. An alternate explanation for the presence of ground-stateand excited-state neutrals, as well as ionized species, along the sameline of sight is a magnetic precursor that heats and compresses the gasahead of the shock.
| IC 2395 and BH 47: Only one open cluster in the Vela constellation
We report UBV photoelectric photometry for 273 stars in the fields ofthe open clusters IC 2395 and BH 47. Our postulate is that, rather thantwo different clusters in this region of Vela, there is only one, whichwe call IC 2395 = BH 47. The cluster is centered at about alpha =8{h} 42fm 5, delta = -48degr 06farcm 8 (2000), itsangular diameter being ~ 19'. The analysis of the photometric datayields 61 probable cluster members and 16 possible members. Photometricmembership probabilities show good agreement with those obtained from aproper motion study for 21 stars in common. The reddening across thecluster appears to be uniform, the mean E(B-V) value being 0.09 +/-0.02. The true distance modulus is (V-MV)0 = 9.52+/- 0.10, corresponding to a distance from the Sun of (800 +/- 40) pcand 48 pc below the Galactic plane. The cluster age, determined byfitting isochrones with core overshooting, turns out to be (6 +/- 2)Myr. There is a strong likelihood that IC 2395 = BH 47 is physicallyconnected to the Vela OB1C association.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant
We present high-resolution (R~75,000), high signal-to-noise (S/N~100) CaII λ3933.663 and Na I λλ5889.951, 5895.924 spectraof 68 stars in the direction of the Vela supernova remnant. The spectracomprise the most complete high-resolution, high S/N optical survey ofearly-type stars in this region of the sky. A subset of the sight lineshas been observed at multiple epochs, 1993/1994 and 1996. Of the 13stars observed twice, seven have spectra revealing changes in theequivalent width and/or velocity structure of lines, most of which arisefrom remnant gas. Such time variability has been reported previously forthe sight lines toward HD 72089 and HD 72997 by Danks & Sembach andfor HD 72127 by Hobbs and coworkers. We have confirmed the ongoing timevariability of these spectra and present new evidence of variability inthe spectra of HD 73658, HD 74455, HD 75309, and HD 75821. We havetabulated Na I and Ca II absorption-line information for the sight linesin our sample to serve as a benchmark for further investigations of thedynamics and evolution of the Vela SNR. Based on observations obtainedat the European Southern Observatory, La Silla, Chile.
| The Distance to the VELA Supernova Remnant
We have obtained high-resolution Ca II and Na I absorption-line spectratoward 68 OB stars in the direction of the Vela supernova remnant. Thestars lie at distances of 190-2800 pc as determined by Hipparcos andspectroscopic parallax estimations. The presence of high-velocityabsorption attributable to the remnant along some of the sight linesconstrains the remnant distance to 250+/-30 pc. This distance isconsistent with several recent investigations that suggest that thecanonical remnant distance of 500 pc is too large.
| 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.
| Radial velocities in three fields along the southern galactic equator
A list of radial velocities for 764 stars is given for three fields inthe Vela-Carina region of the galaxy. They were obtained from GPO-platestaken at La Silla and reduced following Fehrenbach's method.Slit-spectra were collected with the 152 cm-spectrographic telescope atLa Silla, to derive an accurate radial velocity for a sufficient numberof calibration stars: out of the 29 stars, 26 had no formerly publishedvalue. The global motions of 10 to 14 km/s can be considered as normalon the basis of galactic rotation. Some stars, however, show highvelocities, and are therefore marked with one or two asterisks in thetable.
| High-resolution IUE observations of interstellar absorption lines in the VELA supernova remnant
Ultraviolet spectra of 45 stars in the vicinity of the Vela supernovaremnant were recorded by the short-wavelength echelle spectrographaboard the International Ultraviolet Explorer (IUE). Over one-third ofthe stars show interstellar absorption lines at large radial velocities(greater than 60 km/s). The mapping of these high-velocity components inthe sky suggests the motions are chaotic, rather than from a coherentexpansion of the remnant material. In accord with earlier conclusionsfrom Copernicus data, the gas at high velocity exhibits higher thannormal ionization and shows substantially less depletion of nonvolatileelements than normal interstellar material at low velocities. Relativelystrong lines from neutral carbon in the two excited fine-structurestates indicate that the neutral clouds within the remnant have hadtheir pressures enhanced by the passage of the blast wave from thesupernova. Also, the remnant seems to show a significant enhancement inthe abundances of low-velocity Si IV, C IV, and N V over those found inthe general interstellar medium.
| Positions of stars in regions of 14 southern galactic clusters
Positions have been obtained for a total of 3487 stars scattered over 14regions that are centered on each of the southern galactic clusters NGC1981, 2287, 2437, 2451, 2516, 2546, 2547, 2548, 3114, 3532, IC 2391,2395, 2602, and Truempler 10. A frame of reference has been establishedfor each region using ESO Schmidt plates centered on the clusters, witheach plate containing 20-35 measurable Perth 70 stars that are used fordetermining the positions of 200-400 fainter stars within a centralfield of 25 min of arc radius (covering the corresponding 1.5-m plates).
| The VELA star cloud. II - Early-type stars and long-period Cepheids near the VELA pulsar
Intermediate band, H-beta and RI observations of some 200 stars andthree Cepheids in a small region centered on the Vela pulsar have shownthe presence of additional members of the Vela sheet (450 pc), as wellas two more distant associations, Vel OB1 (800 pc) and Vel OB2 (1800pc). Vel OB2, which may be only some 5-million yr old, probably containsSW (23.4 d), RZ (20.4 d), and SX (19.6 d) Vel. All three Cepheids havean overabundance of metals, relative to the sun, if the photometricindices are abundance dependent only. The luminosities of the threeCepheids, plus AH Vel in the Vela sheet and two other southern Cepheidsdiscussed previously, all based on an independent photometric luminositycalibration, do not indicate a need for a major revision of the zeropoint of the period-luminosity relation.
| Photoelectric photometry of late type stars in the direction opposed to galactic rotation
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974A&AS...16..269G&db_key=AST
| UBV sequences in five southern galactic clusters
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