Formation of rings in weak bars : inelastic collisions and star formation.

A 3D computer code for barred galaxies is proposed. The interstellar medium is represented with massive molecular clouds moving in the background axisymmetric plus barlike gravitational potential. The clouds can collide inelastically, loose the kinetic energy and form bound groups. The star formation is also considered. Between 1 and 4 kpc from the galactic centre the molecular ring is formed as the result of combined effects of a weak bar and viscosity. Smaller rings need stronger bars or high pattern rotation speeds.