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Open Clusters as Galactic Disk Tracers. I. Project Motivation, Cluster Membership, and Bulk Three-Dimensional Kinematics
We have begun a survey of the chemical and dynamical properties of theMilky Way disk as traced by open star clusters. In this firstcontribution, the general goals of our survey are outlined and thestrengths and limitations of using star clusters as a Galactic disktracer sample are discussed. We also present medium-resolution (R ~ 15,0000) spectroscopy of open cluster stars obtained with the Hydramulti-object spectrographs on the Cerro Tololo Inter-AmericanObservatory 4 m and WIYN 3.5 m telescopes. Here we use these data todetermine the radial velocities of 3436 stars in the fields of openclusters within about 3 kpc, with specific attention to stars havingproper motions in the Tycho-2 catalog. Additional radial velocitymembers (without Tycho-2 proper motions) that can be used for futurestudies of these clusters were also identified. The radial velocities,proper motions, and the angular distance of the stars from clustercenter are used to derive cluster membership probabilities for stars ineach cluster field using a non-parametric approach, and the clustermembers so identified are used, in turn, to derive the reliable bulkthree-dimensional motion for 66 of 71 targeted open clusters. Thehigh-probability cluster members that we identify help to clarify thecolor-magnitude sequences for many of the clusters, and are primetargets for future echelle resolution spectroscopy as well asastrometric study with the Space Interferometry Mission (SIMPlanetquest).
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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