Introduction

It is known from stellar kinematics in the solar neighbourhood that the velocity dispersions of old populations are significantly higher than those of young populations. This implies a heating rate which seems hard to reconcile with known heating mechanisms such as stellar encounters or interactions between stars and molecular clouds, spiral structure etc… Our information elsewhere in the disc comes from fairly limited and biased samples that include line of sight velocities and, in some cases, distances (Lewis & Freeman 1989). This is insufficient to infer a velocity dispersion profile reliably. In this contribution we use a dynamical model and a large database of radial velocities of OH/IR stars to deduce the velocity dispersions as function of Galactic distance.

The data

Our catalogue of OH/IR stars is a compilation of the 1612 MHz maser surveys by teLintel Hekkert et al. (1989), Eder et al. (1988) and Sivagnanam & le Squeren (1986). The compilation yielded a total of 1600 positions and radial velocities. The positional information for these surveys was taken from the IRAS Point Source Catalogue (PSC).

In order to obtain an approximately constant BC1 (on average BC = 3.4), we selected only those stars from the OH/IR catalogue with an R212 between 0.0 and 0.9. Although the 1612 MHz surveys have been made with different radio telescopes, the catalogue has a well denned detection limit of 3 Jy (12µm), or 8 kpc assuming a luminosity of 5000 L⊙ for a given OH/IR star. We used Habing's (1986) luminosity distribution of the OH/IR stars for our modelling, recalculated for the above R21 range.