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Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
| WIYN Open Cluster Study. XVII. Astrometry and Membership to V=21 in NGC 188
We present techniques for obtaining precision astrometry using oldphotographic plates from assorted large-aperture reflectors incombination with recent CCD Mosaic Imager frames. At the core of thisapproach is a transformation of plate/CCD coordinates into a previouslyconstructed astrometric reference frame around the open cluster NGC 188.This allows us to calibrate independently the optical field angledistortion for all telescopes and field correctors used in this study.Particular attention is paid to computing the differential colorrefraction, which has a marked effect in the case of NGC 188 as a resultof the large zenith distances at which this cluster has been observed.Our primary result is a new catalog of proper motions and positions for7812 objects down to V=21 in the 0.75 deg2 area around NGC188. The precision for well-measured stars is 0.15 mas yr-1for proper motions and 2 mas for positions on the system of the Tycho-2catalog. In total, 1490 stars have proper-motion membershipprobabilities Pμ>=10%. The sum of membershipprobabilities indicates that NGC 188 contains ~1050 stars down to V=21.Comprehensive lists of the candidate blue stragglers and red giant starssubstantially enlarge the number of such stars known in NGC 188. We havealso obtained a small correction to the proper motions from the mean``motion'' of background galaxies. Thus, the absolute proper motion ofNGC 188 is μabsx=-2.56+/-0.2 andμabsy=+0.18+/-0.2 mas yr-1.
| Proper motions, membership, and stellar content of the old cluster NGC 188.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77...74U&db_key=AST
| Photometric Data for the Old Galactic Cluster NGC 188.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1962ApJ...135..333S&db_key=AST
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Цефей |
| Прямое восхождение: | 00h37m30.63s |
| Склонение: | +85°05'33.8" |
| Видимая звёздная величина: | 7.931 |
| Расстояние: | 259.067 парсек |
| Собственное движение RA: | 23.1 |
| Собственное движение Dec: | 6 |
| B-T magnitude: | 9.165 |
| V-T magnitude: | 8.033 |
Каталоги и обозначения:
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