D Wavelet Transform Analysis of Hipparcos Data

A volume limited and absolute magnitude limited sample of A-F type dwarfs, observed by Hipparcos, within 125 parsecs of the sun is searched for in-homogeneities in the phase space, expecting some signatures of the cluster melting and phase mixing mechanisms. The cubes of position and velocity data are analyzed using the discrete wavelet transform implemented by the \\ a trou" algorithm. Sig-niicant wavelet coeecients are identiied through a local hard-thresholding procedure at each scale, assuming a poisson noise. Structures are extracted by a segmentation procedure detecting local maxima and testing the radial decrease of thresholded wavelet co-eecients around each one. In the 3-d distributions of positions not only known open clusters and associations of stars are recognized but two new signiicant concentrations are found. Stars appear well-mixed in position since no more than 7 per cent of them belong to structures. The multi-scale analysis of the 3-d velocity eld, reconstructed by a convergent point method from the two velocity vectors provided by Hipparcos measurements , exhibits the sub-structuring of some kinemat-ics superclusters in smaller velocity clumps. Stars involved in velocity clumps represent 40 per cent of the sample.