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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.
| Third preliminary catalogue of stars observed with the photoelectric astrolabe of the Beijing Astronomical Observatory.
Not Available
| Photometric molecular indices in warm carbon stars - NH, CN, CH, and C2
A ten-color photometric system for measuring band strengths of NH, CN,CH, and C2 molecules is described: the system is capable ofdistinguishing stars with unusual compositions from normal stars, ofquantitatively differentiating among those stars, and of producingresults consistent with spectroscopic data. Measurements for 26 R, CH,CH-like, and barium stars show that the C2 indices for CH and CH-likestars are similar to those of normal R stars. These kinds of stars havemuch stronger C2 indices than normal G-K giants, whereas the classicalbarium stars have C2 indices which are only slightly higher than innormal giants. The NH indices for the classical and marginal bariumstars generally fall within the range of values of NH exhibited bygiants of corresponding temperatures; the NH indices for many of thewarm carbon stars are slightly lower than those for the normal giants.However, statistical analysis of the NH indices for the entire data setshows no significant difference between the warm carbon stars and normalgiants.
| New Determinations of Precise Star Declinations with a Zenith Telescope.
Not Available
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| Csillagkép: | Perszeusz |
| Rektaszcenzió: | 02h40m20.94s |
| Deklináció: | +49°33'36.9" |
| Vizuális fényesség: | 6.596 |
| Távolság: | 476.19 parszek |
| RA sajátmozgás: | 11.5 |
| Dec sajátmozgás: | -8.8 |
| B-T magnitude: | 6.701 |
| V-T magnitude: | 6.605 |
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