The Nature and Orbit of the Ophiuchus Stream
  The Ophiuchus stream is a recently discovered stellar tidal stream in the
Milky Way. We present high-quality spectroscopic data for 14 stream member
stars obtained using the Keck and MMT telescopes. We confirm the stream as a
fast moving ($v_{los}\sim290$ km s$^{-1}$), kinematically cold group
($\sigma_{v_{los}}\lesssim1$ km s$^{-1}$) of $\alpha$-enhanced and metal-poor
stars (${\rm [\alpha/Fe]\sim0.4}$ dex, ${\rm [Fe/H]\sim-2.0}$ dex). Using a
probabilistic technique, we model the stream simultaneously in line-of-sight
velocity, color-magnitude, coordinate, and proper motion space, and so
determine its distribution in 6D phase-space. We find that that the stream
extends in distance from 7.5 to 9 kpc from the Sun; it is 50 times longer than
wide, merely appearing highly foreshortened in projection. The analysis of the
stellar population contained in the stream suggests that it is $\sim12$ Gyr
old, and that its initial stellar mass was $\sim2\times10^4$ $M_{\odot}$ (or at
least $\gtrsim7\times10^3$ $M_{\odot}$). Assuming a fiducial Milky Way
potential, we fit an orbit to the stream which matches the observed phase-space
distribution, except for some tension in the proper motions: the stream has an
orbital period of $\sim350$ Myr, and is on a fairly eccentric orbit
($e\sim0.66$) with a pericenter of $\sim3.5$ kpc and an apocenter of $\sim17$
kpc. The phase-space structure and stellar population of the stream show that
its progenitor must have been a globular cluster that was disrupted only
$\sim240$ Myr ago. We do not detect any significant overdensity of stars along
the stream that would indicate the presence of a progenitor, and conclude that
the stream is all that is left of the progenitor.
