Gaseous orbits in a spiral potential.

We performed MHD simulations of the response of a gaseous disk to a spiral perturbation. We then 
integrated the resulting velocity field in order to obtain the orbits a gas parcel should follow in the perturbed 
potential, and compared those with the orbits stars should follow in the same potential. We found that the 
gaseous orbits does not always correspond to periodic stellar orbits. In fact, the gaseous orbits may not be 
exactly periodic, since the simulation only approaches an equilibrium state but the orbit may show 
dispersion around an orbit near a stellar periodic orbit. Furthermore, a gas parcel may on occasion jump 
between families of orbits.