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The Ophiuchus stream is a short arc-like stellar feature of uncertain originlocated $\sim 5$ kpc North of the Galactic centre. New proper motions from thesecond $Gaia$ data release reconcile the direction of motion of stream memberswith the stream arc, resolving a puzzling mismatch reported in earlier work. Weuse N-body simulations to show that the stream is likely only on its secondpericentric passage, and thus was formed recently. The simulations suggest thatthe entire disrupted progenitor is visible in the observed stream today, andthat little further tidal debris lies beyond the ends of the stream. Theluminosity, length, width, and velocity dispersion of the stream suggest aglobular cluster (GC) progenitor substantially fainter and of lower surfacebrightness than estimated in previous work, and unlike any other knownglobulars in the Galaxy. This result suggests the existence of clusters thatwould extend the known GC population to fainter and more weakly bound systemsthan hitherto known. How such a weakly-bound cluster of old stars surviveduntil it was disrupted so recently, however, remains a mystery. Integratingbackwards in time, we find that the orbits of Sagittarius and Ophiuchus passedwithin $\sim 5$ kpc of each other about $\sim 100$ Myrs ago, an interactionthat might help resolve this puzzle.

The Ophiuchus stream progenitor: a new type of globular cluster and its possible Sagittarius connection