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The outskirts of Cygnus OB2
Context: Cygnus OB2 is one of the richest OB associations in the localGalaxy, and is located in a vast complex containing several otherassociations, clusters, molecular clouds, and HII regions. However, thestellar content of Cygnus OB2 and its surroundings remains rather poorlyknown largely due to the considerable reddening in its direction atvisible wavelength. Aims: We investigate the possible existence of anextended halo of early-type stars around Cygnus OB2, which is hinted atby near-infrared color-color diagrams, and its relationship to CygnusOB2 itself, as well as to the nearby association Cygnus OB9 and to thestar forming regions in the Cygnus X North complex. Methods: Candidateselection is made with photometry in the 2MASS all-sky point sourcecatalog. The early-type nature of the selected candidates is confirmedor discarded through our infrared spectroscopy at low resolution. Inaddition, spectral classifications in the visible are presented for manylightly-reddened stars. Results: A total of 96 early-type stars areidentified in the targeted region, which amounts to nearly half of theobserved sample. Most of them have featureless near-infrared spectra asexpected from OB stars at the available resolution. Another 18 starsthat display Brackett emission lines can be divided between evolvedmassive stars (most likely Be stars) and Herbig Ae/Be stars based ontheir infrared excesses. A component associated with Cygnus OB9/NGC 6910is clearly identified, as well as an enhancement in the surface densityof early-type stars at Cygnus X North. We also find a field population,consisting largely of early B giants and supergiants, which is probablythe same as identified in recent studies of the inner 1° circlearound Cygnus OB2. The age and large extension of this populationdiscards a direct relationship with Cygnus OB2 or any other particularassociation. Conclusions: Earlier claims of the possible large extentof Cygnus OB2 beyond its central, very massive aggregate seem to bedismissed by our findings. The existence of a nearly ubiquitouspopulation of evolved stars with massive precursors suggests a massivestar formation history in Cygnus having started long before theformation of the currently observed OB associations in the region.Based on observations collected at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto, operated jointly by theMax-Planck Institut für Astronomie and the Instituto deAstrofísica de Andalucía (CSIC).Tables 1-9 are only available in electronic form at http://www.aanda.org
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Selection of standard stars for photometric observations with the 91-cm reflector at Okayama
Not Available
| ICCD speckle observations of binary stars. III - A survey for duplicity among high-velocity stars
An analysis of speckle interferometry data for 182 stars has resulted inthe detection of 10 binaries, four of which are newly resolved systems.After correcting for selection effects, the data are found to becompatible with a total frequency for high-velocity long-period doubleswhich is as large as that for low-velocity stars. Based on spectroscopicparallaxes and visual magnitudes, these binaries are found to be within100 pc of the sun, with eight having linear separations of less than 20AU. Four of the binaries are determined to have periods of less than 20yr.
| KURZE MITTEILUNG. Zur Expansion der Assoziation Cep III
Not Available
| Spectral Classification of Stars Noted on Case Objective Prism Plates. II
Not Available
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Cygnus |
| Ascensió Recta: | 20h38m13.12s |
| Declinació: | +39°32'49.5" |
| Magnitud Aparent: | 8.056 |
| Distancia: | 132.802 parsecs |
| Moviment propi RA: | 3.7 |
| Moviment propi Dec: | 29.1 |
| B-T magnitude: | 8.763 |
| V-T magnitude: | 8.115 |
Catàlegs i designacions:
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