Separatrix Divergence of Stellar Streams in Galactic Potentials
  Flattened axisymmetric galactic potentials are known to host minor orbit
families surrounding orbits with commensurable frequencies. The behavior of
orbits that belong to these orbit families is fundamentally different than that
of typical orbits with non-commensurable frequencies. We investigate the
evolution of stellar streams on orbits near the boundaries between orbit
families (separatrices) in a flattened axisymmetric potential. We demonstrate
that the separatrix divides these streams into two groups of stars that belong
to two different orbit families, and that as a result, these streams diffuse
more rapidly than streams that evolve elsewhere in the potential. We utilize
Hamiltonian perturbation theory to estimate both the timescale of this effect
and the likelihood of a stream evolving close enough to a separatrix to be
affected by it. We analyze two prior reports of stream-fanning in simulations
with triaxial potentials, and conclude that at least one of them is caused by
separatrix divergence. These results lay the foundation for a method of mapping
the orbit families of galactic potentials using the morphology of stellar
streams. Comparing these predictions with the currently known distribution of
streams in the Milky Way presents a new way of constraining the shape of our
Galaxy's potential and distribution of dark matter.
