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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}
| The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey. I. X-Ray Properties of Clusters Detected as Extended X-Ray Sources
In the construction of an X-ray-selected sample of galaxy clusters forcosmological studies, we have assembled a sample of 495 X-ray sourcesfound to show extended X-ray emission in the first processing of theROSAT All-Sky Survey. The sample covers the celestial region withdeclination δ>=0deg and Galactic latitude|bII|>=20deg and comprises sources with a countrate >=0.06 counts s-1 and a source extent likelihood ofL>=7. In an optical follow-up identification program we find 378(76%) of these sources to be clusters of galaxies. It was necessary toreanalyze the sources in this sample with a new X-ray sourcecharacterization technique to provide more precise values for the X-rayflux and source extent than obtained from the standard processing. Thisnew method, termed growth curve analysis (GCA), has the advantage overprevious methods in its ability to be robust, to be easy to model and tointegrate into simulations, to provide diagnostic plots for visualinspection, and to make extensive use of the X-ray data. The sourceparameters obtained assist the source identification and provide moreprecise X-ray fluxes. This reanalysis is based on data from the morerecent second processing of the ROSAT Survey. We present a catalog ofthe cluster sources with the X-ray properties obtained as well as a listof the previously flagged extended sources that are found to have anoncluster counterpart. We discuss the process of source identificationfrom the combination of optical and X-ray data. To investigate theoverall completeness of the cluster sample as a function of the X-rayflux limit, we extend the search for X-ray cluster sources to the dataof the second processing of the ROSAT Survey for the northern sky regionbetween 9h and 14h in right ascension. We includethe search for X-ray emission of known clusters as well as a newinvestigation of extended X-ray sources. In the course of this search wefind X-ray emission from 85 additional Abell clusters and 56 veryprobable cluster candidates among the newly found extended sources. Acomparison of the X-ray cluster number counts of the NORAS sample withthe ROSAT-ESO Flux-limited X-Ray (REFLEX) Cluster Survey results leadsto an estimate of the completeness of the NORAS sample of ROSAT All-SkySurvey (RASS) I extended clusters of about 50% at an X-ray flux ofFX(0.1-2.4 keV)=3×10-12 ergs s-1cm-2. The estimated completeness achieved by adding thesupplementary sample in the study area amounts to about 82% incomparison to REFLEX. The low completeness introduces an uncertainty inthe use of the sample for cosmological statistical studies that will becured with the completion of the continuing Northern ROSAT All-Sky(NORAS) Cluster Survey project. Results reported here are based onobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
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Observation and Astrometry data
| Constellation: | Hercules |
| Right ascension: | 17h30m18.64s |
| Declination: | +22°02'07.8" |
| Apparent magnitude: | 7.671 |
| Distance: | 172.414 parsecs |
| Proper motion RA: | -10.4 |
| Proper motion Dec: | 23.8 |
| B-T magnitude: | 8.693 |
| V-T magnitude: | 7.756 |
Catalogs and designations:
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