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

Abstract

Dynamical self-consistent constraints deduced from the Boltzmann equation are necessary to analyse and interpret kinematic data in the Galaxy. It has been shown that the standard epicycle theory approach, a first order theory to estimate galactic characteristics, is subject to systematic errors. In some recent work, distribution function solutions of the Boltzmann equation and/or third moments in Jeans equations have been used to obtain a more consistent description of our Galaxy. In this paper, we give a short review of observational constraints and the different values usualy taken for global parameters describing our Galaxy, namely the slope of the circular velocity curve, the density and kinematic scale lengths. Secondly we build a new model of Shu-type distribution functions where scale lengths and the shape of the velocity curve are free parameters. Thirdly we compare and adjust this model to velocity distributions of stars in the solar neighbourhood taken from the Gliese catalogue and other ones. Implicit constraints are related to the asymmetric drift relation and the Lindblad equation that gives the ratio of radial and tangential velocity dispersions. We conclude that extended Shu distribution functions recover most of the fundamental kinematic properties of the galactic disc. We show that it is possible to build a consistent model explaining local stellar kinematics and to determine local structural galactic parameters.