Local Kinematics of K and M from CORAVEL/Hipparcos/Tycho 2 data

Abstract

The availability of the Hipparcos Catalogue has triggered many kinematic and dynamical studies ofthe solar neigh-bourhood. Nevertheless, those studies generally lacked the third component of the space velocities, i.e., the radial velocities.This work presents the kinematic analysis of 5952 K and 739 M giants in the solar neighbourhood which includes for the firsttime radial velocity data from a large survey performed with the CORAVEL spectrovelocimeter. It also uses proper motionsfrom the Tycho-2 catalogue, which are expected to be more accurate than the Hipparcos ones. An important by-product of thisstudy is the observed fraction of only 5.7% of spectroscopic binaries among M giants as compared to 13.7% for K giants. Afterexcluding the binaries for which no center-of-mass velocity could be estimated, 5311 K and 719 M giants remain in the finalsample.TheUV-plane constructed from these data for the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scalestructure, with several clumps corresponding to the Hercules stream, the Sirius moving group, and the Hyades and Pleiadessuperclusters. A maximum-likelihood method, based on a 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 in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these groups. Thesegroups are most probably related to the dynamical perturbation by transient spiral waves (as recently modelled by De Simoneet al. 2004) rather than to cluster remnants. A possible explanation for the presence of young group/clusters in the same area oftheUV-plane is that they have been put there by the spiral wave associated with their formation, while the kinematics of theolder stars of our sample has also been disturbed by the same wave. The emerging picture is thus one ofdynamical streamspervading the solar neighbourhood and travelling in the Galaxy with similar space velocities. The termdynamical streamismore appropriate than the traditional termsuperclustersince it involves stars of different ages, not born at the same place norat the same time. The position of those streams in theUV-plane is responsible for the vertex deviation of 16.2◦±5.6◦for thewhole sample. Our study suggests that the vertex deviation for younger populations could have the samedynamicalorigin.The underlying velocity ellipsoid, extracted by the maximum-likelihood method after removal of the streams, is not centeredon the value commonly accepted for the radial antisolar motion: it is centered on〈U〉=−2.78±1.07 km s−1.However,thefull data set (including the various streams)doesyield the usual value for the radial solar motion, when properly accountingfor the biases inherent to this kind of analysis (namely,〈U〉=−10.25±0.15 km s−1). This discrepancy clearly raises theessential question of how to derive the solar motion in the presence of dynamical perturbations altering the kinematics of thesolar neighbourhood: does there exist in the solar neighbourhood a subset of stars having no net radial motion which can beused as a reference against which to measure the solar motion?