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Astron. Astrophys. 325, 149-158 (1997)

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1. Introduction

The existence of moving groups or superclusters of stars in the solar neighbourhood has been known for over one century (Eggen 1989). Nevertheless, their study has been largely restricted to the descriptive aspects of their stellar contents, probably due at least in part to the fact that their very definition as swarms of stars moving with near parallel velocities is fundamentally phenomenological. On the other hand, increasing research in the recent years has been devoted to the study of large star complexes in our and other galaxies, and their relationship to the large scale processes of star formation in galactic disks (Efremov & Chernin 1994). An inmediate question is therefore whether the moving groups in our solar neighbourhood may be related to these larger complexes, and how we may use the detailed knowledge about the kinematics of nearby stars to gain some insight on the formation and evolution of larger structures in galactic disks or about the recent history of star formation in the solar neighbourhood. In this respect, attempts have been made to trace back the orbits of member stars of moving groups to determine their birthplaces (Grosbol 1976, Yuan 1977, Yuan & Waxman 1977, Palous & Hauck 1986), or to interpret them in terms of bursts of star formation (Gómez et al. 1990), former clusters disrupted by the close encounter with a massive molecular cloud (Wielen 1985), or dispersing coronae of open clusters (Agekian & Belozerova 1979, Mülläri et al. 1994).

A property of some of the best studied moving groups in the solar neighbourhood, such as the Hyades and the Ursae Majoris groups, and, although less pronouncedly, the Pleiades group, is the deviation of the motion of their member stars from the purely circular velocity. Such deviations in fact make their recognition possible, as their members occupy regions of the space velocity diagram falling on the wings of the velocity distribution of field stars; see for instance Fig. 2 in Palous & Hauck 1986. The peculiar velocities of these moving groups are well above the velocity dispersion of field stars with comparable ages. It is interesting to wonder whether the large spatial velocities of the stars may be related to the initial conditions of their formation, as a consequence of some energetic mechanism involved both in triggering the formation of the member stars and in determining their further kinematical behavior.

One such possible mechanism, which we will consider in this paper, is related to the existence of large scale shocks in the interstellar gas associated to the spiral structure of the Galaxy. The aim of this paper is to suggest an interpretative framework for at least some of the moving groups based on the existence of spiral shocks. In doing this, we will suggest a redefinition of what may be understood by a moving group in the context of our model, which in practice is equivalent to the classical definition when one is restricted to study samples limited to a small volume around the Sun. We will also provide some predictions that can be tested in the near future, when higher precission velocities and distances for stars belonging to these moving groups are available thanks to the data obtained by the Hipparcos satellite.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998