6 Conclusions

  A systematic multi-scale analysis of both the space and velocity distributions of a thin disc young star sample has been performed. Over-densities in space distributions are mainly well known open clusters (Hyades OCl, Coma OCl and Ursa Major OCl) and associations of stars (Centaurus-Crux and Centaurus-Lupus). Evidence is given for three nearby loose clusters or associations not detected yet (Bootes and Pegasus 1 and 2). Evaporation of relatively massive stars (1.8 $M_{\odot}$) out of the Hyades open cluster is also mapped in Paper II and form an asymmetric pattern. The sample is well mixed in position space since no more than 7$\%$ of the stars are in concentrated clumps with coherent tangential velocities.

The 3D velocity field reconstructed from a statistical convergent point method exhibits a strong structuring at typical scales of $\overline{\sigma}_{\rm stream}\sim$ 6.3, 3.8 and 2.4 km s^-1. At large scale (scale 3) the majority of structures are Eggen's superclusters (Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association embedded in a larger kinematic structure. A new supercluster-like structure is found with a mean velocity between the Sun and Sirius SCl velocities. These large scale velocity structures are all characterized by a large age range which reflects the overall sample age distribution. Moreover, few old streams of $\sim$ 2 Gyr are also extracted at this scale with high U components towards the Galactic center. Taking into account the fraction of spurious members, evaluated with an observed radial velocity data set, into all these large velocity dispersion structures we show that they 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 ($\overline{\sigma}_{\rm stream}\sim$ 3.8 and 2.4 km s^-1) reveal that superclusters are always substructured by 2 or more streams which generally exhibits a coherent age distribution. The older the stream is the more difficult the age segregation between close velocity clumps is. At scale 2 and 1, background stars are negligible and percentages of stars in streams, after evaluating the fraction of spurious members, are 38$\%$ and 18$\%$respectively.

Acknowledgements

This work was facilitated by the use of the Vizier service and the Simbad database developed at CDS. E.C thanks A. Bijaoui and J.L. Starck for constructive discussions on wavelet analysis, and C. Pichon for interesting discussions and help on some technical points. E.C is very grateful to R. Asiain for providing the two codes of age calculation from Strömgren photometry and thanks J. Torra and F. Figueras for valuable discussions.