We present a progress report on the dynamical analysis of the IRAS sample of OH/IR stars. This sample is complete within a distance of 8 kpc. We distinguish two groups of OH/IR stars, one with high and the other with low expansion velocity of the circumstellar shell.
Using a quadratic programming method we fit a database of Galactic orbits to the observed distribution of radial velocities, longitudes and latitudes. The dynamical model yields a distribution function, based on two integrals of motion, for each group. Integration gives the density, the mean velocity and the velocity dispersions as function of R and z. The distribution functions for the two groups differ enough to suggest that they represent two different stellar populations.
We estimate the stellar lifetime for each group by comparing the velocity dispersions and z scale-heights at the solar radius with those found for local stellar populations. We conclude that the group with the low expansion velocity is likely to be part of the thick disc, while the high expansion velocity group has dynamical characteristics resembling an old disc population; this in contrast with the findings for differently selected samples of OH/IR stars (e.g., Baud et al. 1981a; 1981b), which were found to be less than 1 Gyr old.