Spending too much time at the Galactic bar: chaotic fanning of the
  Ophiuchus stream
  The Ophiuchus stellar stream is peculiar: (1) its length is short given the
age of its constituent stars, and (2) several probable member stars that lie
close in both sky position and velocity have dispersions in these dimensions
that far exceed those seen within the stream. The stream's proximity to the
Galactic center suggests that the bar must have a significant influence on its
dynamical history: The triaxiality and time-dependence of the bar may generate
chaotic orbits in the vicinity of the stream that can greatly affect its
morphology. We explore this hypothesis with models of stream formation along
orbits consistent with Ophiuchus' properties in a Milky Way potential model
that includes a rotating bar. We find that in all choices for the rotation
parameters of the bar, orbits fit to the stream are strongly chaotic. Mock
streams generated along these orbits qualitatively match the observed
properties of the stream: because of chaos, stars stripped early generally form
low-density, high-dispersion "fans" leaving only the most recently disrupted
material detectable as a strong over-density. Our models predict that there
should be more low-surface-brightness tidal debris than detected so far, likely
with a complex phase-space morphology. The existence of or lack of these
features around the Ophiuchus stream would provide an interesting constraint on
the properties of the Milky Way bar and would help distinguish between
formation scenarios for the stream. This is the first time that chaos has been
used to explain the properties of a stellar stream and is the first
demonstration of the dynamical importance of chaos in the Galactic halo. The
existence of long, thin streams around the Milky Way---presumably formed along
non- or weakly-chaotic orbits---may represent only a subset of the total
population of disrupted satellites.
