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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.
| The multiplicity of exoplanet host stars. Spectroscopic confirmation of the companions GJ 3021 B and HD 27442 B, one new planet host triple-star system, and global statistics
Aims.We present new results from our ongoing multiplicity study ofexoplanet host stars and present a list of 29 confirmed planet hostmultiple-star systems. Furthermore, we discuss the properties of thesestellar systems and compare the properties of exoplanets detected inthese systems with those of planets orbiting single stars. Methods: Weused direct imaging to search for wide stellar and substellar companionsof exoplanet host stars. With infrared and/or optical spectroscopy, wedetermined the spectral properties of the newly-found co-movingcompanions. Results: We obtained infrared H- and K-band spectra of theco-moving companion GJ 3021 B. The infrared spectra and the apparentH-band photometry of the companion is consistent with an M3-M5 dwarf atthe distance of the exoplanet host star. HD 40979 AB is a wide planethost stellar system, with a separation of ~ 6400 AU. The companion tothe exoplanet host star turned out to be a close stellar pair with aprojected separation of ~130 AU, hence, this system is a new member ofthose rare planet host triple-star systems of which only three othersystems are presently known. HD 27442 AB is a wide binary system listedin the Washington Double Star Catalogue, whose common proper motion wasrecently confirmed. This system is composed of the subgiant HD 27442 Ahosting the exoplanet, and its faint companion HD 27442 B. The visibleand infrared J-, H-, and K_S-band photometry of HD 27442 B at thedistance of the primary star shows that the companion is probably awhite dwarf. Our multi-epochs SofI imaging observations confirm thisresult and even refine the suggested physical characteristics of HD27442 B. This companion should be a relatively young, hot white dwarfwith an effective temperature of ~14 400 K, and cooling age of ~220 Myr.Finally, we could unambiguously confirm the white dwarf nature of HD27442 B with follow-up optical and infrared spectroscopy. The spectra ofthe companion show Hydrogen absorption features of the Balmer, Paschen,and Bracket series. With its subgiant primary and the white-dwarfcompanion, the HD 27442 AB system is the most evolved planet hoststellar system known today. The mass-period and eccentricity-periodcorrelation of planets around single stars and those residing inmultiple-star systems seem different for the short-period planets. Thedistribution functions of planet orbital elements (P, e) are identical,while the mass-distribution (m sin(i)) exhibits one difference. Whileboth planet populations exhibit a peak in their mass-distribution atabout 1 M_Jup, the frequency of more massive planets continuallydecreases in single-star systems, whereas the mass-distribution ofplanets residing in multiple-star systems exhibits a further peak atabout 4 M_Jup. This indicates that the mass-distributions of the twoplanet populations might differ in the intermediate mass-range between 2and 6 M_Jup.Based on observations obtained on La Silla in ESO programs 70.C-0116(A),71.C-0140(A), 73.C-0103(A), and on Paranal in ESO runs 074.C-0144(B),074.C-0144(C), 073.C-0370(A), on Mauna Kea in UKIRT program U/02A/16, aswell as at the Munich LMU University Observatory on Mount Wendelstein.
| 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.
| 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 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.
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