Effect of the drag force on the orbital motion of the broad-line region
  clouds
  We investigate orbital motion of cold clouds in the broad line region of
active galactic nuclei subject to the gravity of a black hole and a force due
to a nonisotropic central source and a drag force proportional to the velocity
square. The intercloud is described using the standard solutions for the
advection-dominated accretion flows. Orbit of a cloud decays because of the
drag force, but the typical time scale of falling of clouds onto the central
black hole is shorter comparing to the linear drag case. This time scale is
calculated when a cloud is moving through a static or rotating intercloud. We
show that when the drag force is a quadratic function of the velocity,
irrespective of the initial conditions and other input parameters, clouds will
generally fall onto the central region much faster than the age of whole system
and since cold clouds present in most of the broad line regions, we suggest
that mechanisms for continuous creation of the clouds must operate in these
systems.
