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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.
| 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.
| Optical observations of ultraviolet objects. I - Spectral classification of 103 stars /l = 200-275 deg/
Results are presented of a program of spectral classification of 103stars originally selected as ultraviolet objects from TD-1 satellitephotometry with the S2/68 experiment. Most of the objects appear to bespectroscopically normal stars; the method of selection yielded a sampleof relatively unreddened B stars at distances up to about 2 kpc. Thisresult is compared with recent studies of the spatial distribution ofinterstellar extinction in the same regions of the sky.
| The region of NGC 2287 and CR 121
Intermediate band and H-beta observations of 135 stars in the regions ofthe clusters NGC 2287 and Cr 121 are discussed, and a luminositycalibration of photometric parameters for late G- to early K-type brightgiants and supergiants is introduced. Results indicate that NGC 2287 isat a distance of 740 pc, very little reddened, 100-million years old,and contains three or four G8-K2 bright giants and supergiants and ablue straggler. Cr 121 is 1.17 kpc distant, very little reddened, 1.5million years old and an extension of CMa OB1. Cr 121 contains a nearerconcentration of stars at the same distance as NGC 2287, and theassociation appears to be the same age as CMa OB1, although presequencestars may exist. Intermingling of stars in CMa OB2 and NGC 2287 is alsoconsidered possible, and a test of the calibration of two methods ofluminosity determination of early A-type stars using photometricparameters shows them to be entirely consistent.
| H-beta photometry of southern early-type stars
H-beta photoelectric photometry is presented for 209 southern hemisphereearly-type stars from the HD catalog with galactic latitudes /b/ greaterthan 6 deg. Four-color photometry exists for all these stars and MKtypes for most of them. Absolute magnitudes have been estimated for allbut the emission-line stars and distances from the sun and the galacticplane determined.
| Four colour photometry of southern early-type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978MNRAS.182..629K&db_key=AST
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Observation and Astrometry data
| Constellation: | Grand Chien |
| Right ascension: | 06h56m04.34s |
| Declination: | -21°30'21.3" |
| Apparent magnitude: | 9.181 |
| Proper motion RA: | -3.1 |
| Proper motion Dec: | 4.7 |
| B-T magnitude: | 9.059 |
| V-T magnitude: | 9.171 |
Catalogs and designations:
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