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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.
| The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy
We present further arguments that the Hipparcos parallaxes for some ofthe clusters and associations represented in the Hipparcos catalogshould be used with caution in the study of the Galactic structure. Ithas already been shown that the discrepancy between the Hipparcos andground-based parallaxes for several clusters including the Pleiades,Coma Ber, and NGC 6231 can be resolved by recomputing the Hipparcosastrometric solutions with an improved algorithm diminishing correlatederrors in the attitude parameters. Here we present new parallaxesobtained with this algorithm for another group of stars with discrepantdata-the galactic cluster Cr 121. The original Hipparcos parallaxes ledde Zeeuw et al. to conclude that Cr 121 and the surrounding associationof OB stars form a relatively compact and coherent moving group at adistance of ~=550-600 pc. Our corrected parallaxes reveal a differentspatial distribution of young stellar populace in this area. Both thecluster Cr 121 and the extended OB association are considerably moredistant (750-1000 pc), and the latter has a large depth probablyextending beyond 1 kpc. Therefore, not only are the recalculatedparallaxes in complete agreement with the photometric uvbyβparallaxes, but the structure of the field they reveal is no longer indiscrepancy with that found by the photometric method.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The region of Collinder 121
The distribution of bright B-type stars in a field with a radius of5° centred at the Galactic open cluster Cr 121 is studied utilizingStrömgren and Hβ photometry. All PPM stars earlier thanspectral type A0 are used, revealing a loose nearby structure at adistance of 660-730pc, and a compact more distant group, which appearsto be a genuine cluster: Cr 121. Based on similar coordinates, distancesand positions on the colour-magnitude (CM) and Hertzsprung-Russell (HR)diagrams, 11 photometric cluster members are selected at a mean distanceof 1085(+/-41 standard error) pc. The results are discussed in the lightof both classical and Hipparcos points of view.
| 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
| 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.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| The evolution of low ionization QSO absorption systems
A CCD spectroscopic survey of Mg II and Mg I absorption lines in a C IVselected sample of 35 metal systems, observed toward 12 QSOs between z =1.1 and z = 2.1, is reported. Six Mg I and 10 Mg II absorption lines aredetected in 33 C IV clouds, and some peculiarities are noted. Thecomplex or asymmetric line profiles reveal that multiple components arepresent and that variations in the C IV/Mg II and Mg II/Mg I ratio mayoccur often from one component to another with a velocity separationless than the velocity resolution of about 100 km/s. Comparison with theMg II absorption-line statistics at z-bar = 0.5 implies that, at z-bar =1.6, there are more strong absorbers per unit z with W(2796) greaterthan 0.6 A and fewer weak absorbers. The evolutionary parameter gammaappears to depend on W(min): its value drops as W(min) is reduced and isnegative at W(min) = 0.15 A. This suggests a different redshiftevolution for the population of weak and strong Mg II absorbers.
| The ultraviolet spectrum and interstellar environment of HD 50896
Data from 92 high-resolution IUE 1150-3274-A spectra are compiled intables and graphs and analyzed in detail to characterize HD 50896 andits interstellar environment. Findings reported include unexceptionaldepletions in the low-velocity H I system, strong low-velocity lines(attributed to excitation in an H II region associated with HD 50896 atdistance 2-3 kpc), an absorption system blueshifted by about 30 km/sfrom the low-velocity gas (attributed to ring nebula S308), andhigh-velocity absorption systems with evidence of grain destruction(attributed to an old SNR).
| High-velocity interstellar gas in the line of sight to the Wolf-Rayet star HD 50896
The large shell of interstellar gas (IG) discovered toward HD 50896 byHeckathorn and Fesen (1984) is characterized on the basis ofhigh-dispersion IUE SWP and LWR spectra of 19 objects located within 4deg of HD 50896 (but outside the optical ring nebula S308) at distances0.6-2.9 kpc (compared to 1.5 kpc for HD 50896). The IG is found to havetwo components (at velocities -80 and -125 km/s), diameter 90 pc orgreater, and distance 1.0 + or - 0.2 kpc, demonstrating that it is notrelated to HD 50896 and suggesting that it is a highly evolved supernovaremnant associated with cluster Cr 121.
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Observation and Astrometry data
| Constellation: | Grand Chien |
| Right ascension: | 06h51m26.42s |
| Declination: | -25°22'09.6" |
| Apparent magnitude: | 8.78 |
| Distance: | 286.533 parsecs |
| Proper motion RA: | -0.8 |
| Proper motion Dec: | 2.7 |
| B-T magnitude: | 8.621 |
| V-T magnitude: | 8.767 |
Catalogs and designations:
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INHABITED SKY
