The Ophiuchus stream progenitor: a new type of globular cluster and its
  possible Sagittarius connection
  The Ophiuchus stream is a short arc-like stellar feature of uncertain origin
located $\sim 5$ kpc North of the Galactic centre. New proper motions from the
second $Gaia$ data release reconcile the direction of motion of stream members
with the stream arc, resolving a puzzling mismatch reported in earlier work. We
use N-body simulations to show that the stream is likely only on its second
pericentric passage, and thus was formed recently. The simulations suggest that
the entire disrupted progenitor is visible in the observed stream today, and
that little further tidal debris lies beyond the ends of the stream. The
luminosity, length, width, and velocity dispersion of the stream suggest a
globular cluster (GC) progenitor substantially fainter and of lower surface
brightness than estimated in previous work, and unlike any other known
globulars in the Galaxy. This result suggests the existence of clusters that
would extend the known GC population to fainter and more weakly bound systems
than hitherto known. How such a weakly-bound cluster of old stars survived
until it was disrupted so recently, however, remains a mystery. Integrating
backwards in time, we find that the orbits of Sagittarius and Ophiuchus passed
within $\sim 5$ kpc of each other about $\sim 100$ Myrs ago, an interaction
that might help resolve this puzzle.
