PROGRAM OF THE GALACTIC MERIDIAN - STAR COUNTS AND GALACTIC MODELS .1. DISTRIBUTIONS OF STELLAR MAGNITUDES AND COLORS

A comparison of observed stellar distributions with a three-component model of the Galaxy is presented. The analysis is based on photometric and photoelectric data obtained along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The assum ed model considers the Galaxy as composed of the disk (main sequence a nd disk red giants), the thick disk and spheroid populations. To model the observed colour distribution, we distinguish main sequence stars and disk red giants as the disk subsystem; white dwarfs, subdwarfs and intermediate giants as the thick disk subsystem; extreme subdwarfs, s pheroid giants and horizontal branch stars as the spheroid subsystem. A statistical relation between the apparent and absolute magnitudes of stars which make the maximum contribution to the star counts for a gi ven disk subsystem is derived. In order to achieve the best agreement between the model and observations, we fit the values of the 'dip' (al pha(w)) of the disk luminosity function, the correction to the absolut e magnitude of disk red giants (Delta M(V)(RG)) and the expression for interstellar extinction. As the main result, we obtained alpha(w) = 0 .6 (logarithmic scale) and Delta M(V)(RG) = 0.5 mag; the interstellar extinction has to be taken into account by the modified Sandage law.