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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}
| A 2572 and HCG 94 - galaxy clusters but not as we know them: an X-ray case study of optical misclassifications
We present the results of a spectral-imaging analysis of X-ray dataobtained with the Position Sensitive Proportional Counter aboard theROSAT Observatory in a 32-ks pointed observation of Hickson's CompactGroup (HCG) £94. Besides HCG 94, A2572, a richness class 0Abell cluster, is also contained in the central region of the field ofview. Both systems are at a redshift of z~0.04 and are falling towardeach other at a velocity of about 1000 km s^-1. Their three-dimensionalspatial separation is probably of the order of an Abell radius; however,as yet, no clear signs of dynamical interaction are discernible in theX-ray data. We find HCG 94's gas temperature and unabsorbed X-rayluminosity to be far too high for a galaxy group, thereby confirming theclaim of Ebeling, Voges & Bohringer that HCG 94 should be classifiedas a galaxy cluster. The opposite is true for the Abell cluster A2572,the optical richness of which has been overestimated due to theinclusion of HCG 94. In the X-ray, A2572 appears at first sight like atypical binary cluster with two equally massive and X-ray-brightsubclusters in the process of merging. However, the available X-ray,optical, and radio data strongly suggest that A2572 proper is, in fact,merely a loose group of galaxies, while the second component is a muchricher and more distant cluster seen in superposition. A deprojectionanalysis shows HCG 94 to host a moderate cooling flow; this picture issupported by a radial increase in the column density of absorbingmaterial and a decrease in the gas temperature toward the clustercentre. HCG 94's total gravitating mass is much higher than what couldbe anticipated from its appearance in the optical. Our findings henceunderline the need for X-ray-selected cluster samples. For all threeclusters studied in this paper we find the baryon fraction to rise withradius and reach values of 15 to 30 per cent at the outer edge of ourstudy regions. If any of these values is to be taken as representativeof the overall baryon fraction of the Universe, then this resultrequires the latter to be open with Omega_0<0.35 if a conflict withthe baryon density derived from nucleosynthesis calculations is to beavoided.
| Photoelectric radial velocities, Paper XIII - 406 ninth-magnitude K0 stars in the Clube Selected Areas
Radial velocities are given for 406 ninth-magnitude late-type stars. Thestars are grouped in 10 small regions located on a systematic plan atGalactic latitudes of + or - 35 deg. Each star has been observed atleast twice, and the mean velocities are accurate to about less than 1km/s. Thirty spectroscopic binaries have been discovered; orbits havealready been given for 13 of them, and observations of the others arecontinuing.
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Osservazione e dati astrometrici
| Costellazione: | Pegaso |
| Ascensione retta: | 23h17m54.20s |
| Declinazione: | +18°42'08.6" |
| Magnitudine apparente: | 8.843 |
| Distanza: | 395.257 parsec |
| Moto proprio RA: | -14.5 |
| Moto proprio Dec: | -10.7 |
| B-T magnitude: | 10.274 |
| V-T magnitude: | 8.962 |
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INHABITED SKY
