Periodic orbits and gas flow in barred spirals

A Henyey method for finding families of periodic orbits has been applied to the bar potential used by Sanders & Tubbs (1980) in twodimensional hydrodynamical calculations. Families of periodic orbits have been found in the limits of weak and strong bars and compared qualitatively to the results of Contopoulos & Papayannopoulos (1980). Orbits in the strong bar potential are compared to the gas flow pattern determined in the earlier hydrodynamical calculations. It is shown that the pattern in which periodic orbits crowd can account for the linear off-set shocks found in the gas dynamical calculations provided that the bar is strong, that the mass distribution is centrally condensed and that the bar figure is rapidly tumbling.