The distribution of nearby stars in phase space mapped by Hipparcos - II. Inhomogeneities among A-F type stars

A volume limited and absolute magnitude limited sample of A-F type dwarfs w ithin 125 parsecs of the Sun is searched for inhomogeneities in the density -velocity space, expecting signatures of the cluster evaporation, phase mix ing and possible disc heating mechanisms. A 3-D wavelet analysis is used to extract inhomogeneities, both in the density and velocity distributions. T hus, a real picture of the phase space is produced. Not only are some clust ers and streams detected, but the fraction of clumped stars can be measured . By estimating individual stellar ages one can relate the streams and clus ters to the state of the interstellar medium (ISM) at star formation time a nd provide a quantitative view of cluster evaporation and stream mixing. As a result, we propose a coherent interpretation of moving groups or so-call ed superclusters and derive some quantitative evolutionary tracers which we expect to serve in the understanding of the large scale evolution of the g alactic 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 measu rements. Then, the 3D velocity field is reconstructed from a statistical co nvergent point method. The wavelet analysis exhibits strong velocity struct uring at typical scales of velocity dispersion <(sigma)over bar>(stream) si milar to 6.3, 3.8 and 2.4 km s(-1). The majority of large scale velocity st ructures (<(sigma)over bar>(stream) similar to 6.3 km s(-1)) are Eggen's su perclusters. As illustrated by the Pleiades supercluster these structures a re 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 ba ckground expected by a smooth velocity ellipsoid in each structure. Smaller scales (<(sigma)over bar>(stream) similar to 3.8 and 2.4 km s(-1)) reveal that superclusters are always substructured by 2 or more streams which gene rally exhibit a coherent age distribution. At these scales, the contributio n of background stars is negligible and percentages of stars in streams are 38% and 18% respectively. The detailed analysis of the phase space structu res provides a scenario of kinematical evolution in the solar neighbourhood : star formation in the galactic disc occurs in large bursts (possibly subd ivided into smaller bursts) separated by quiescent periods. The velocity sp ace is gradually populated by these star formation bursts which preferentia lly fill the center of the velocity ellipsoid. Stars form in groups reflect ing the clumpy structure of the ISM: about 75% of recently formed stars bel ong 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 gravit ationally bound and form open clusters. Open clusters sustain a longer term streaming with quite similar velocity by an evaporation process due to int ernal processes or encounters with permanent or transient large mass concen trations. These streams are detected with <(sigma)over bar>(stream) similar to 2.3 and 3.8 km s(-1) and have a coherent age content. This process expl ains the survival of streams up to 10(9) yr. The existence of streams as ol d as 2 Gyr seems to require other physical mechanisms. The typical scale of so-called Eggen's superclusters (<(sigma)over bar>(stream) similar to 6.3 km s(-1)) does not seem to correspond to any physical entity. The picture t hey form, their frequency and their divisions at smaller scales are well co mpatible with their creation by chance coincidence of physically homogeneou s smaller scale structures (<(sigma)over bar>(stream) similar to 3.8 or 2.4 km s(-1)).