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Astron. Astrophys. 323, 781-788 (1997)

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Stellar kinematics in the solar neighbourhood and the disc scale lengths of the Galaxy

O. Bienaymé and N. Séchaud

Observatoire de Strasbourg, CNRS URA 1280, 11 rue de l'Université, F-67000 Strasbourg, France

Received 7 May 1996 / Accepted 17 January 1997


A general dynamically consistent 2D flat distribution function is built to model the kinematics of neighbouring stars. Application leads to the measurement of a short galactic scale length [FORMULA] between 1.7 and [FORMULA] and a locally decreasing rotation curve. This is in agreement with recent determinations based on kinematics and counts of distant stars. These results rule out the classical assumption that [FORMULA] or that [FORMULA] is constant when the vertical scale height [FORMULA] is assumed to be constant. We explain why the measured squared axis ratio of the velocity dispersions [FORMULA] of disc stars is less than [FORMULA]. This ratio has been claimed to be important evidence for the non-axisymmetry of the galactic disc. We show that this is not the case and that it may be simply explained with a realistic axisymmetric disc model if the circular velocity is locally declining or if there is a mismatch between the photometric and kinematic scale lengths.

Key words: stars: kinematics – Galaxy: kinematics and dynamics – solar neighbourhood – Galaxy: structure

Send offprint requests to: O. Bienaymé

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998