Dynamics of gas and dust clouds in active galactic nuclei
  We analyse the motion of single optically thick clouds in the potential of a
central mass under the influence of an anisotropic radiation field
~|cos(\theta)|, a model applicable to the inner region of active galactic
nuclei. Resulting orbits are analytically soluble for constant cloud column
densities. All stable orbits are closed, although they have non-trivial shapes.
Furthermore, there exists a stability criterion in the form of a critical
inclination, which depends on the luminosity of the central source and the
column density of the cloud.
