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Astron. Astrophys. 350, 434-446 (1999)

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6. Conclusions

In this paper we studied the disruption of unbound systems of stars as a mechanism to understand the origin and evolution of moving groups. The epicycle theory was used to find an analytic expression for the time dependence of dispersion in both the stellar position and velocity coordinates. We have obtained in this way a simple expression for the secular increase of the dispersion in the azimuthal galactic coordinate over time as a function of the initial conditions. This increase in dispersion is, in fact, a direct consequence of the galactic differential rotation.

In order to overtake the constraints of the epicycle theory, we concentrated our analysis on the determination of the stellar trajectories using numerical integration of the equations of motion, which provided us with an independent and more accurate estimation of the evolution of unbound systems. To perform this integration we used an analytic and axisymmetric galactic potential, along with spiral arm and central bar perturbations. This latter procedure allowed us to include random perturbations that mimic the disc heating effect on stellar trajectories.

The trajectories followed by the members of the Pleiades moving group substructures found in Paper I were used to compare the kinematic and photometric ages of these structures, and to establish their position at birth. The youngest group, B1 (the Sco-Cen association), was found to be most spatially concentrated some [FORMULA] yr ago, a value considerably smaller than its photometric age. This is probably due to the effect of the high stellar atmospheric rotation of early type stars on the observed photometric colors. Groups B4 and B3 were born at a time from the present equal to one and two times the epicycle period respectively, which means that they are spatially focused at present (probably that is the reason why we can observe them). At birth, these two groups were found to be close to the spiral arm structure. Concerning B2, a detailed analysis revealed to us that it is actually composed of several associations which are disintegrating at present. As well, its averaged photometric age is probably overstimated, as in the case of B1.

We considered the evolution of a stellar complex under the influence of the galactic gravitational potential as a mechanism to account for the main physical properties of moving groups. The high velocity dispersions of some of the Pleiades moving group substructures detected among B and A type stars could be recovered when the effect of the disc heating on the individual stellar trajectories was considered. At the same time, the disc heating can account for the mixing of the stellar complex associations, although they are still clearly separated in phase space during the first tens of million years in the complexes' life (as observed for the groups B1 and B2). After only [FORMULA] yr from the birth of the stellar complex, the complex occupies an ellipsoidal area of [FORMULA] kpc2, with its longest axis oriented in the direction of galactic rotation. If the Sun were presently located in the very center of this disrupted stellar complex, we would be able to find the complex's components up to very large distances on the galactic plane.

Thus, a constant heating mechanism (compatible with the observational heating law) acting on the stars of a stellar complex can explain the velocity dispersions obtained for those Pleiades moving group substructures that are younger than [FORMULA]. The properties of the older Pleiades substructures could probably be recovered by considering a diffusion coefficient depending on the velocity of the stars, and maybe on the time, and/or the inclusion of open clusters in the simulated stellar complex.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999