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Description
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_~290km/s), kinematically cold group ({sigma}_{nu}_los__<~1km/s) of {alpha}-enhanced and metal-poor stars ([{alpha}/Fe]~0.4dex, [Fe/H]~-2.0dex). 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 9kpc 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 ~12Gyr old, and that its initial stellar mass was ~2x10^4^M_{sun}_ (or at least >~7x10^3^M_{sun}_). 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 ~350Myr, and is on a fairly eccentric orbit (e~0.66) with a pericenter of ~3.5kpc and an apocenter of ~17kpc. The phase-space structure and stellar population of the stream show that its progenitor must have been a globular cluster that was disrupted only ~240Myr 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.
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