The galactic structure towards the galactic centre. II. A study of the fields near the clusters NGC 6603, Lynga 7 and Terzan 1

The results are presented from a study of Colour-Magnitude Diagrams in three fields, located in the galactic plane towards the Galactic Centre. First the extinction is determined in the line of sight. In two fields a significant increase in the absorption is found. For the field near Terzan 1 the increase starts immediately at 0 kpc in the line of sight and for the field near NGC 6603 it occurs between 3.5 kpc and 4.0 kpc. The latter apparently coincides with the position of the molecular ring. The abrupt end in the steep increment in the absorption is likely the result of gas consumption by star formation. In two fields a significant increase in the number of stars with probably a contribution from a stellar ring has been found. The stellar ring can be described with a Gaussian centered around 3.5 kpc from the Galactic Centre with sigma(ring) = 0.5 kpc. Between 1.0 to about 3.0 kpc in the line of sight a stellar contribution is present from the Sagittarius spiral arm. In the fields near the clusters Lynga 7 and Terzan 1 relatively young stars are present from a recent burst of star formation. The field of NGC 6603 crosses a patchy region in this arm where this recent burst is absent. In all fields the major contribution for V < 22(m) is due to disc stars. Around 2.5 kpc from the Galactic Centre we find a sharp drop in the stellar density of the disc. The stellar density towards the Galactic Centre is still increasing due to the stellar contribution from the Bulge. Therefore we identify this as the disc/bulge transition. For the Bulge we suggest the presence of a super metal rich population.