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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 ROSAT pointed observation of the Chamaeleon II dark cloud
A deep 13.5 ksec ROSAT PSPC pointed observation in the Chamaeleon II(Cha II) cloud is reported. 40 X-ray sources are detected of which 14can be identified with previously known young stellar objects (YSOs),namely IRAS sources, classical T Tauri stars and weak T Tauri stars.From spectroscopic follow-up observations, four new weak T Tauricandidates have been found. The X-ray sources are mainly located on thenorth-east of the cloud and their spatial distribution follows the lanesof the 100mu m dust emission. Their X-ray properties are similar tothose of low-mass PMS stars. None of the protostar candidates in Cha IIhas been detected in the ROSAT pointed observation, in agreement withthe ASCA observations results. The X-ray detection rates indicate thatthe weak T Tauri stars (WTTS) are less numerous than the classical TTauri stars (CTTS), contrarily to the findings in Chamaeleon I (Cha I)and other star forming regions where the WTTS may outnumber the CTTS.The latter result could be a consequence of the fact that Cha II is inan earlier evolutionary stage as compared to Cha I, as conjectured byprevious studies. The Cha II young stellar objects (YSOs) are, on theaverage, slightly less X-ray luminous than those in Cha I, but thenormalised X-ray luminosity distribution functions of the two regionsare not significantly different. Based on observations with the EuropeanSouthern Observatory, La Silla, Chile under proposal number 55.E-0792
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Observation and Astrometry data
| Constellation: | Ursa Major |
| Right ascension: | 13h04m40.00s |
| Declination: | +53°51'27.3" |
| Apparent magnitude: | 7.347 |
| Distance: | 338.983 parsecs |
| Proper motion RA: | -13.4 |
| Proper motion Dec: | 17.5 |
| B-T magnitude: | 9.041 |
| V-T magnitude: | 7.487 |
Catalogs and designations:
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