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 (vlos ˜ 290 km s-1), kinematically cold group ({σ }{v{los}}≲ 1 km s-1) of α-enhanced and metal-poor stars ([α/Fe] ˜ 0.4 dex, [Fe/H] ˜ -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 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 ˜12 Gyr old, and that its initial stellar mass was ˜2 × 104 M⊙ (or at least ≳7 × 103 M⊙). Assuming a fiducial Milky Way potential, we fit an orbit to the stream that matches the observed phase-space distribution, except for some tension in the proper motions: the stream has an orbital period of ˜350 Myr, and is on a fairly eccentric orbit (e ˜ 0.66) with a pericenter of ˜3.5 kpc and an apocenter of ˜17 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 ˜240 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.