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Astron. Astrophys. 340, 384-396 (1998)

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

II. Inhomogeneities among A-F type stars

E. Chereul 1, M. Crézé 1, 2 and O. Bienaymé 1

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


A volume limited and absolute magnitude limited sample of A-F type dwarfs within 125 parsecs of the Sun is searched for inhomogeneities in the density-velocity space, expecting signatures of the cluster evaporation, phase mixing and possible disc heating mechanisms. 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. As a result, we propose a coherent interpretation of moving groups or so-called superclusters and derive some quantitative evolutionary tracers which we expect to serve in the understanding of the large scale evolution of the galactic disc.

The sample is relatively well mixed in the position space since less than 7 per cent of the stars are proper motion confirmed cluster members. We also detect star evaporation out of the Hyades open cluster.

Only two components of the velocity vectors are provided by Hipparcos measurements. Then, the 3D velocity field is reconstructed from a statistical convergent point method. The wavelet analysis exhibits strong velocity structuring at typical scales of velocity dispersion [FORMULA] 6.3, 3.8 and 2.4 km s-1. The majority of large scale velocity structures ([FORMULA] 6.3 km s-1) are Eggen's superclusters . As illustrated by the Pleiades supercluster these structures are all characterized by a large age range which reflects the overall sample age distribution. These large velocity dispersion structures represent 63% of the sample. This percentage drops to 46% if we subtract the velocity background expected by a smooth velocity ellipsoid in each structure. Smaller scales ([FORMULA] 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, the contribution of background stars is negligible and percentages of stars in streams are 38% and 18% respectively. The detailed analysis of the phase space structures provides a scenario of kinematical evolution in the solar neighbourhood: star formation in the galactic disc occurs in large bursts (possibly subdivided into smaller bursts) separated by quiescent periods. The velocity space is gradually populated by these star formation bursts which preferentially fill the center of the velocity ellipsoid. Stars form in groups reflecting the clumpy structure of the ISM: about 75% of recently formed stars belong to streams whose internal velocity dispersions do not exceed 4 km s-1. Most of them dissolve rapidly. A fraction of the initial groups are gravitationally bound and form open clusters. Open clusters sustain a longer term streaming with quite similar velocity by an evaporation process due to internal processes or encounters with permanent or transient large mass concentrations. These streams are detected with [FORMULA] 2.3 and 3.8 km s-1 and have a coherent age content. This process explains the survival of streams up to [FORMULA] yr. The existence of streams as old as 2 Gyr seems to require other physical mechanisms. The typical scale of so-called Eggen's superclusters ([FORMULA] 6.3 km s-1) does not seem to correspond to any physical entity. The picture they form, their frequency and their divisions at smaller scales are well compatible with their creation by chance coincidence of physically homogeneous smaller scale structures ([FORMULA] 3.8 or 2.4 km s-1).

Key words: methods: data analysis – Galaxy: kinematics and dynamics – Galaxy: open clusters and associations: general – Galaxy: solar neighbourhood

* Based on data from the Hipparcos astrometry satellite

Send offprint requests to: chereul@cdsxb6.u-strasbg.fr

© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998