The distribution of nearby stars in phase space mapped by Hipparcos^*

III. Clustering and streaming among A-F type stars

¹ Centre de Données de Strasbourg, Observatoire Astronomique de Strasbourg, 11 rue de l'Université, F-67000 Strasbourg, France
² I.U.P de Vannes, Tohannic, rue Yves Mainguy, F-56000 Vannes, France

Received: 21 April 1998
Accepted: 21 September 1998

Abstract

This paper presents the detailed results obtained in the search of density-velocity inhomogeneities in a volume limited and absolute magnitude limited sample of A-F type dwarfs within 125 parsecs of the Sun. A 3-D wavelet analysis is used to extract inhomogeneities, both in the density and velocity distributions. Thus, a real picture of the phase space is produced. Not only are some clusters and streams detected, but the fraction of clumped stars can be measured. By estimating individual stellar ages one can relate the streams and clusters to the state of the interstellar medium (ISM) at star formation time and provide a quantitative view of cluster evaporation and stream mixing. Having established this picture without assumption we come back to previously known observational facts regarding clusters and associations, moving groups or so-called superclusters. In the 3-D position space, well known open clusters (Hyades, Coma Berenices and Ursa Major), associations (parts of the Scorpio-Centaurus association) as well as the Hyades evaporation track are retrieved. Three new probably loose clusters are identified (Bootes, Pegasus 1 and 2). The sample is relatively well mixed in the position space since less than 7 per cent of the stars belong to structures with coherent kinematics, most likely gravitationally bound. The wavelet analysis exhibits strong velocity structuring at typical scales of velocity dispersion ~ 6.3, 3.8 and 2.4 km s^-1. The majority of large scale velocity structures ( ~ 6.3 km s^-1) are Eggen's superclusters (Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association. A new supercluster-like structure is found with a mean velocity between the Sun and Sirius SCl velocities. These structures are all characterized by a large age range which reflects the overall sample age distribution. Moreover, a few old streams of ~ 2 Gyr are also extracted at this scale with high U components. We show that all these large velocity dispersion structures represent 63% of the sample. This percentage drops to 46% if we remove the velocity background created by a smooth velocity ellipsoid in each structure. Smaller scales ( 3.8 and 2.4 km s^-1) reveal that superclusters are always substructured by 2 or more streams which generally exhibit a coherent age distribution. At these scales, background stars are negligible and percentages of stars in streams are 38% and 18% respectively.