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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}
| Distances of absorbing features in the LDN 1622 direction. An application of Tycho-2 photometry and Michigan Classification
With Tycho-2 BT and VT photometry of Hipparcosstars with π > 8.0 mas, σπ/π < 15% andwith spectral type and luminosity classification from the MichiganCatalogues we have a presumably unreddened sample for establishing a(BT-VT_0- MV_T main sequence relationin the range from A0 to G5. We discuss the location of the median mainsequence relation relative to published ZAMS relations for the A0-G5range in some detail since the applications of this relation will haveto assume that the evolutionary status of the calibration sample isrepresentative of the local disk. If the ZAMS is defined as the lowerenvelope of the class V stars the published versions we discuss seem tobe too bright by about half a magnitude in the F0-G5 range. Theresulting intrinsic color and absolute magnitude estimates have errorsof the mean of 0.03m and 0.3m-0.4m respectively. From this relation anygiven star with Tycho-2 photometry, spectral classification andluminosity class V can have its color excess E(BT -VT) and distance modulus estimated. This means that the localinterstellar extinction may be estimated for the ~50% of the sky whereMichigan classification presently is available. The individual colorexcesses and distance moduli may not be particularly accurate but wepropose that they may be used to indicate extinction discontinuitiescaused by interstellar material at well defined distances. We haveapplied this new technique on stars from the Michigan Catalogue in thedirection of LDN 1622 (l, b) = (204.7dg, -11.8dg) supposed to beassociated to the Orion B region 400-500 pc away but color excessesE(BT - VT) exceeding 0.15m start appearing alreadyin the distance slot 160-200 pc. Presently we cannot decide whether thisnearby dust is related to LDN 1622 or whether this cloud is associatedto Orion B. In a final comparison we study the color excess -- distancevariation in a 4 deg region centered on LDN 1622 from Hipparcos --Tycho-1 -- Michigan data and the dust at ~160 pc is confirmed.
| Star-streaming from the proper motion of the A-type stars of the Boss General Catalogue
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1954MNRAS.114..593T&db_key=AST
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