THE EVOLUTION OF THE MILKY-WAY DISC .2. CONSTRAINTS FROM STAR COUNTS AT THE GALACTIC POLES

Classical models of the stellar populations in our Galaxy based on exp onential density laws turn out to be unsuitable to match comprehensive ly recent data from deep surveys at a broad variety of wavelengths. Ac tually, we show that such models may generate spurious interpretations of deep star-count data, resulting in biaised estimates of the densit y characteristics of each population. Instead, a fully synthetic appro ach must take into account the time dependence of observable star prop erties as well as the dynamical constraints relating age, kinematics a nd space distributions. We present here comparisons of colour magnitud e star-count data with the predictions of models in which the age of d isc stars works as an explicit parameter connecting the star's intrins ic properties with the dynamical properties of the generation it belon gs to. Hence, the history of star formation, the initial mass function , and the heating rate of the stellar disc can be tested against their respective impact on star-count data. Star counts at the galactic pol e combined with the more local data investigated in Paper I strongly f avour star formation scenarios with constant or increasing SFR, while those implying a maximum of star formation activity in the early disc followed by a substantial decrease are excluded. The slope of the IMF in the mass range 1 to 3M. is found between 1.8 and 2.0 (associated wi th a constant SFR). Such an IMF does not produce efficient gas recycli ng, as a result the gas infall required to key the SFR nearly constant must be about 3.5 M..pc(-2).Gyr(-1). The vertical density distributio n of disc stars decreases much faster than the conventionnal 300-350 p c exponential scale height and simple valued exponentials used in clas sical models are not an adequate description of the density laws. Inst ead, the dynamical consistent density laws adopted in the Besancon mod el do fit available magnitude and colour distributions of stars over a very wide range of magnitudes. It is also shown that the thick disc c ontribution to faint star counts has been heavily underestimated. The whole dynamical equilibrium and fit to local and non local data is obt ained under a scenario involving a moderate contribution of unseen mat ter to the local potential (local surface density of less than 8 M..pc (-2)) and rather limited increase of the velocity dispersion of disc s tars with age (sigma(w) < 21 km.s(-1) reached after more than 3 Gyrs).