Evidence of Fanning in the Ophiuchus Stream
  The Ophiuchus stellar stream presents a dynamical puzzle: its old stellar
populations ($\sim 12$ Gyr) cannot be reconciled with (1) its orbit in a simple
model for the Milky Way potential and (2) its short angular extent, both of
which imply that the observed stream formed within the last $<1$ Gyr. Recent
theoretical work has shown that streams on chaotic orbits may abruptly fan out
near their apparent ends; stars in these fans are dispersed in both position
and velocity and may be difficult to associate with the stream. Here we present
the first evidence of such stream-fanning in the Ophiuchus stream, traced by
four blue horizontal branch (BHB) stars beyond the apparent end of the stream.
These stars stand out from the background by their high velocities ($v_{\rm
los} > 230$ km s$^{-1}$) against $\sim 40$ other stars: their velocities are
comparable to those of the stream, but would be exceptional if they were
unrelated halo stars. Their positions and velocities are, however, inconsistent
with simple extrapolation of the observed cold, high-density portion of the
stream. These observations suggest that stream-fanning may be a real,
observable effect and, therefore, that Ophiuchus may be on a chaotic orbit.
They also show that the Ophiuchus stream is more extended and hence dynamically
older than previously thought, easing the stellar population vs. dynamical age
tension.
