Stray, swing and scatter: angular momentum evolution of orbits and
  streams in aspherical potentials
  In aspherical potentials orbital planes continuously evolve. The
gravitational torques impel the angular momentum vector to precess, that is to
slowly stray around the symmetry axis, and nutate, i.e. swing up and down
periodically in the perpendicular direction. This familiar orbital pole motion
- if detected and measured - can reveal the shape of the underlying
gravitational potential, the quantity only crudely gauged in the Galaxy so far.
Here we demonstrate that the debris poles of stellar tidal streams show a very
similar straying and swinging behavior, and give analytic expressions to link
the amplitude and the frequency of the pole evolution to the flattening of the
dark matter distribution. While these results are derived for near-circular
orbits, we show they are also valid for eccentric orbits. Most importantly, we
explain how the differential orbital plane precession leads to the broadening
of the stream and show that streams on polar orbits ought to scatter faster. We
provide expressions for the stream width evolution as a function of the
axisymmetric potential flattening and the angle from the symmetry plane and
prove that our models are in good agreement with streams produced in N-body
simulations. Interestingly, the same intuition applies to streams whose
progenitors are on short or long-axis loops in a triaxial potential. Finally,
we present a compilation of the Galactic cold stream data, and discuss how the
simple picture developed here, along with stream modelling, can be used to
constrain the symmetry axes and flattening of the Milky Way.
