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Astron. Astrophys. 341, 86-97 (1999)


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The local stellar velocity distribution of the Galaxy *

Galactic structure and potential

O. Bienaymé

CDS, Observatoire Astronomique de Strasbourg, 11 rue de l'Université, F-67000 Strasbourg, France

Received 18 June 1998 / Accepted 2 October 1998

Abstract

The velocity distribution of neighbouring stars is deduced from the Hipparcos proper motions. We have used a classical Schwarzschild decomposition and also developed a dynamical model for quasi-exponential stellar discs. This model is a 3-D derivation of Shu's model in the framework of Stäckel potentials with three integrals of motion.

We determine the solar motion relative to the local standard of rest (LSR) ([FORMULA], [FORMULA] and [FORMULA]), the density and kinematic radial gradients, as well as the local slope of the velocity curve. We find out that the scale density length of the Galaxy is [FORMULA]. We measure a large kinematic scale length for blue (young) stars, [FORMULA], while for red stars (predominantly old) we find [FORMULA] (or [FORMULA]).

From the stellar disc dynamical model, we determine explicitly the link between the tangential-vertical velocity [FORMULA] coupling and the local shape of the potential. Using a restricted sample of 3-D velocity data, we measure [FORMULA], the focus of the spheroidal coordinate system defining the best fitted Stäckel potential. The parameter [FORMULA] is related to the tilt of the velocity ellipsoid and more fundamentally to the mass gradient in the galactic disc. This parameter is found to be [FORMULA]. This implies that the galactic potential is not extremely flat and that the dark matter component is not confined in the galactic plane.

Key words: stars: kinematics – Galaxy: fundamental parameters – Galaxy: kinematics and dynamics – Galaxy: solar neighbourhood – Galaxy: structure – cosmology: dark matter

* Based on data from the Hipparcos astrometry satellite

Send offprint requests to: O. Bienaymé

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998

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