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Astron. Astrophys. 351, 506-518 (1999)

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1. Introduction

The most distinctive and intriguing feature of the Gould Belt system of young stars and star forming regions is its tilt of [FORMULA] with respect to the galactic equator. Several models for the origin of the Gould Belt exist in the literature, such as those based on the fragmentation of a large molecular complex, on propagating star formation triggered by supernovae, the compression of gas during the passage of a spiral density wave, or the collision of high velocity gas with the galactic disk (see Pöppel 1997for a comprehensive review). However, few of these models have explicitly addressed the question of the origin of the tilt. Observations of the distribution of the stars in the Gould Belt show that the tilt is preserved at least over the distance from [FORMULA] Ophiuchi to the Orion molecular complex, the two star forming regions which delimit the extent of the Belt in the galactocentric and antigalactocentric directions (roughly coinciding with its apsidal line), implying a coherence over a lengthscale of about 700 pc. This coherence, and the very existence of the Gould Belt as a single entity, have been eventually called into question by authors who noted that the distribution of stars in the Belt is apparently dominated by a few major structures which might have independent origins, and whose arrangement along a tilted disk may be casual (Franco et al. 1988, Lépine & Duvert 1994; see also Guillout et al. 1998). Indeed, most of the early-type stars that make the Gould Belt outstanding belong to the OB associations of Orion, Perseus OB2, and Scorpius-Centaurus-Lupus (Blaauw 1991, de Zeeuw et al. 1999). However, the existence of a distributed population of young stars not associated with any of the massive star forming complexes is revealed for instance by the sky distribution of young, chromospherically active low mass stars detected in the ROSAT all-sky survey (Neuhäuser 1997, Guillout et al. 1998). Moreover, the precise three-dimensional picture of the stellar distribution in the solar neighbourhood provided by the Hipparcos satellite clearly shows the existence of a distributed population of B stars that depict the Gould Belt as a disk with its members spread well outside the boundaries of the known OB associations. As shown by Torra et al. 1999, the kinematical peculiarities distinctive of the Gould Belt are preserved even when the stars belonging to the dominant associations are excluded from analyisis. On the other hand, the overall age of the Gould Belt, although very uncertain (published estimates range between 20 and 90 million years; see Torra et al. 1999for a review of determinations found in the literature), is in any case at least a considerable fraction of the vertical oscillation period of stars around the galactic plane under the effects of the galactic gravitational potential, implying that the coherent structure of the Belt applies not only to the position of its components, but to their motions as well.

In this paper I intend to make a schematic exploration on the implications that the maintenance of the coherence with time of the structure of the Gould Belt has when combined to its kinematics, stressing the importance of stellar motions perpendicular to the galactic plane when interpreting measured velocity gradients. I will first present evidence for a systematic pattern of the vertical components of stellar motions, revealed by the Hipparcos data. Using the epicyclic approximation to the orbits of the stars in the galactic potential, I will then discuss the conditions under which an ensemble of stars, having an initial pattern of peculiar motions and being distributed on a tilted plane, can still define a plane as their trajectories evolve. The time evolution of the tilted plane with respect to the galactic equator, in particular its inclination, the position of its nodal line, and the different velocity gradients expected from its member stars, will be studied under the assumption of different initial patterns of motion. Finally, a global interpretation of the Gould Belt kinematics based on the actually observed orientation and velocity patterns will be discussed.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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