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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}
| HR 1614 and the dissolution of a supercluster
A few main-sequence members of the HR 1614 supercluster have been foundwithin 40 parsecs of the sun. The convergent point of the proper motionsis (A,D) = (8.38 h - 58.7 deg) and V(TOT) = 59.0 km/s. Thirty-ninemembers of HR 1614 group, with mean V = 58.4 km/s, have been found nearthe sun as well as nineteen red giants within 200 parsecs. Both thesupercluster and the group members have Hyades abundance of heavyelements but CN strengths equalled by less than five percent of thestars near the sun. The age of the supercluster and group stars is nearthat of those in the cluster M67, 5 x 10 exp 9 yr on the basis of modelswithout convective overshoot or 6.3 x 10 exp 9 yr for models withovershoot. The stellar density near the sun is similar for both Hyadessupercluster and the combined HR 1614 supercluster and group members.
| Space motions of galactic G- and K-type stars
Spectra of 912 G- and K-type stars in six galactic directions have beensecured. The relevant space motions of the stars, subdivided intodistance groups, have been derived; these show no significant departuresfrom the mean motion of nearby stars. The velocity dispersions, ingeneral, indicate that these (mainly giant) stars are moderately old.The dispersion in u is lower than that for nearby stars, and thesignificance of this result is discussed.
| A sample of old-disk-population red giants.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976PASP...88..426E&db_key=AST
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Osservazione e dati astrometrici
| Costellazione: | Pegaso |
| Ascensione retta: | 22h37m55.77s |
| Declinazione: | +29°55'25.6" |
| Magnitudine apparente: | 7.277 |
| Distanza: | 148.148 parsec |
| Moto proprio RA: | 71.6 |
| Moto proprio Dec: | -22.6 |
| B-T magnitude: | 8.867 |
| V-T magnitude: | 7.409 |
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