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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.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| The EXOSAT high Galactic latitude survey
This study presents a survey of serendipitous sources performed in thevery soft X-ray band (0.05-2.0 keV) using the Exosat imaging telescopes.It covers 783 sq deg of high Galactic latitude sky and includes 210serendipitous sources which define a complete (flux-limited) sample. Twohundred of the 210 detected sources are identified via extensive opticaland radio observations together with cross-correlations with catalogs ofknown objects. The log N-log S relation was found to be consistent withthat of the Einstein extended Medium-Sensitivity Survey (EMSS) and withthe 'Euclidean' value of 1.5. The normalization of the relation is astrong function of the assumed spectral slope of AGN. It is inferredfrom the consistency with the EMSS results that the average (energy)slope of extragalactic sources in the soft X-ray band is very steep(approximately 1.5). An analysis of the association between AGNdetection and Galactic N(H) also shows that the average slope is steepand inconsistent with the canonical value of 0.7.
| Meridian observations made with the Carlsberg Automatic Meridian Circle at Brorfelde (Copenhagen University Observatory) 1981-1982
The 7-inch transit circle instrument with which the present position andmagnitude catalog for 1577 stars with visual magnitudes greater than11.0 was obtained had been equipped with a photoelectric moving slitmicrometer and a minicomputer to control the entire observationalprocess. Positions are reduced relative to the FK4 system for each nightover the whole meridian rather than the usual narrow zones. Thepositions of the FK4 stars used in the least squares solution are alsogiven in the catalog.
| A magnitude limited stellar X-ray survey and the F star X-ray luminosity function
An X-ray survey has been conducted of stars brighter than visualmagnitude 8.5 that have serendipitously fallen into the fields of viewof the Imaging Proportional Counter of the Einstein Observatory. Thesurvey includes 227 separate 1 x 1 deg fields, containing 274 stars witha visual magnitude of no more than 8.5 and covering a wide range ofspectral types and luminosity classes. X-ray emission was detected from33 stars, and upper limits have been determined for the remainder of thesample. F type stars dominate the detected sample, and most of these areshown to be dwarfs. An X-ray luminosity function for dF stars has beendeduced, and reveals that the average 0.2-4.0 keV luminosity of thesestars is around 10 to the 29th erg/sec. Constraints have been placed onthe high luminosity tails and medians of the X-ray luminosity functionsfor other types of stars.
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Observation and Astrometry data
| Constellation: | Taurus |
| Right ascension: | 04h31m50.45s |
| Declination: | +05°45'52.0" |
| Apparent magnitude: | 6.67 |
| Distance: | 196.078 parsecs |
| Proper motion RA: | 15.5 |
| Proper motion Dec: | -20.9 |
| B-T magnitude: | 6.667 |
| V-T magnitude: | 6.67 |
Catalogs and designations:
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