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

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6. Summary

The main results of the present work can be summarized as follows:

  • There is a systematic gradient in the vertical component of the velocity of the stars belonging to the Gould Belt along the galactic plane, subtle but detectable in the Hipparcos astrometric data. Such a gradient amounts of [FORMULA] km s- 1 kpc-1, a rather small value that suggests that the Gould Belt is at present near its maximum tilt.

  • The pattern of such vertical motions implies an instantaneous axis of oscillation around the galactic plane oriented along a line forming an angle of [FORMULA] with the direction of the nodal line in which the Gould Belt intersects the galactic plane.

  • The maintenance of the distribution of the Gould Belt stars on a plane for a time that is comparable to the vertical oscillation period around the galactic plane can be achieved if the initial pattern of motions has a linear dependence with their initial positions.

  • Analytical expressions can be found for the evolution of the orientation of the Gould Belt, the properties of the vertical velocities of their stars, and the Oort constants as a function of time under the assumption of a linear dependence between the initial positions and velocities and the validity of the epicyclic approximation to galactic orbits.

  • Such evolution has been considered for different cases, whose initial parameters have been chosen so as to fit the presently observed orientation of the Gould Belt and its nearly maximum tilt.

  • Comparison between model results and measured parameters seem to rule out kinematical models of the Gould Belt in which the stars initially expand away either from a point or from a line.

  • A model in which the stars of the Gould Belt are initially rotating around an axis perpendicular to the plane of the Belt, and then move independently following their epicyclic orbits, achieves the best match to the observations, simultaneosly fitting the orientation of the Gould Belt, the Oort constants A, B, C, and K, the gradient in the vertical component of the velocity, and the offset between the axis of vertical oscillation and the direction of the nodal line. Such a best fit implies an age of [FORMULA] years for the Gould Belt.

  • It seems unlikely, in view of actual observations of giant molecular clouds, that the formation of the Gould Belt can be simply explained by the dissolution of such a complex, due to the requirement of a significant misalignment between its rotation axis and the direction perpendicular to the galactic plane. The impact of a high velocity cloud with the galactic disk as the formation mechanism of the Belt may provide a more adequate explanation.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999