The Nature and Orbit of the Ophiuchus Stream

Branimir Sesar, Jo Bovy, Edouard J. Bernard, Nelson Caldwell, Judith G. Cohen, Morgan Fouesneau, Christian I. Johnson, Melissa Ness, Annette M. N. Ferguson, Nicolas F. Martin, Adrian M. Price-Whelan, Hans-Walter Rix, Edward F. Schlafly, William S. Burgett, Kenneth C. Chambers, Heather Flewelling, Klaus W. Hodapp, Nick Kaiser, Eugene A. Magnier, Imants PlataisJohn L. Tonry, Christopher Waters, Rosemary F. G. Wyse

Research output: Contribution to journalArticlepeer-review


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.
Original languageEnglish
Pages (from-to)59
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 10 Aug 2015


  • Galaxy: halo
  • Galaxy: kinematics and dynamics
  • Galaxy: structure
  • globular clusters: general

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