Origin and evolution of moving groups

I. Characterization in the observational kinematic-age-metallicity space

Departament d'Astronomia i Meteorologia and IEEC-UB, Institut de Ciències del Cosmos de la Universitat de Barcelona, Martí i Franquès, 1, 08028 Barcelona, Spain e-mail: tantoja@am.ub.es

Received: 5 February 2008
Accepted: 5 August 2008

Abstract

Context. Recent studies have suggested that moving groups have a dynamic or “resonant” origin. Under this hypothesis, these kinematic structures become a powerful tool for studying the large-scale structure and dynamics of the Milky Way.

Aims. Here we aim to characterize these structures in the U–V-age-[Fe/H] space and establish observational constraints that will allow us to study their origin and evolution.

Methods. We apply multiscale techniques – wavelet denoising (WD) – to an extensive compendium of more than 24 000 stars in the solar neighbourhood with the best available astrometric, photometric and spectroscopic data.

Results. We confirm that the dominant structures in the U–V plane are the branches of Sirius, Coma Berenices, Hyades-Pleiades and Hercules. These branches are nearly equidistant in this kinematic plane and they show a negative slope. The abrupt drops in the velocity density distribution are characterized. We find a certain dependence of these kinematic structures on Galactic position with a significant change of contrast among substructures inside the branches. A large spread of ages is observed for all branches. The Hercules branch is detected in all subsamples with ages older than ~2  and the set of the other three branches is well established for stars >400 . The age-metallicity relation of each branch is examined and the relation between kinematics and metallicity is studied.

Conclusions. Not all of these observational constraints are successfully explained by the recent models proposed for the formation of such kinematic structures. Simulations incorporating stellar ages and metallicities are essential for future studies. The comparison of the observed and simulated distributions obtained by WD will provide a physical interpretation of the existence of the branches in terms of local or large-scale dynamics.