Astron. Astrophys. 351, 506-518 (1999)
The main results of the present work can be summarized as
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
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 with the direction of
the nodal line in which the Gould Belt intersects the galactic
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
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
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.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999