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Published August 10, 2015 | Submitted + Published
Journal Article Open

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 (v_(los) ~ 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 × 10^4 M⊙ (or at least ≳7 × 10^3 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.

Additional Information

© 2015 The American Astronomical Society. Received 2015 January 3; accepted 2015 June 25; published 2015 August 10. B.S. acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP 7) ERC Grant Agreement n. [321035]. C.I.J. gratefully acknowledges support from the Clay Fellowship, administered by the Smithsonian Astrophysical Observatory. A.P.W. is supported by a National Science Foundation Graduate Research Fellowship under Grant No. 11-44155. The Pan-STARRS1 Surveys (PS1) have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), and the Los Alamos National Laboratory. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. Facilities: PS1 - Panoramic Survey Telescope and Rapid Response System Telescope #1 (Pan-STARRS), Keck:I (DEIMOS) -, MMT (Hectochelle) - MMT at Fred Lawrence Whipple Observatory.

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Published - Sesar_2015.pdf

Submitted - 1501.00581v2.pdf


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