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Published October 10, 2013 | Submitted + Published
Journal Article Open

Tracing the Orphan Stream to 55 kpc with RR Lyrae Stars


We report positions, velocities and metallicities of 50 ab-type RR Lyrae (RRab) stars observed in the vicinity of the Orphan stellar stream. Using about 30 RRab stars classified as being likely members of the Orphan stream, we study the metallicity and the spatial extent of the stream. We find that RRab stars in the Orphan stream have a wide range of metallicities, from -1.5 dex to -2.7 dex. The average metallicity of the stream is -2.1 dex, identical to the value obtained by Newberg et al. (2010) using blue horizontal branch stars. We find that the most distant parts of the stream (40-50 kpc from the Sun) are about 0.3 dex more metal-poor than the closer parts (within ~ 30 kpc), suggesting a possible metallicity gradient along the stream's length. We have extended the previous studies and have mapped the stream up to 55 kpc from the Sun. Even after a careful search, we did not identify any more distant RRab stars that could plausibly be members of the Orphan stream. If confirmed with other tracers, this result would indicate a detection of the end of the leading arm of the stream. We have compared the distances of Orphan stream RRab stars with the best-fit orbits obtained by Newberg et al. (2010). We find that model 6 of Newberg et al. (2010) cannot explain the distances of the most remote Orphan stream RRab stars, and conclude that the best fit to distances of Orphan stream RRab stars and to the local circular velocity is provided by potentials where the total mass of the Galaxy within 60 kpc is M_(60) ~ 2.7 x 10^(11) M_⊙, or about 60% of the mass found by previous studies. More extensive modelling that would consider non-spherical potentials and the possibility of misalignment between the stream and the orbit, is highly encouraged.

Additional Information

© 2013 American Astronomical Society. Received 2013 June 30; accepted 2013 August 8; published 2013 September 23. B.S and J.G.C thank NSF grant AST-0908139 to J.G.C for partial support, as do S.R.K (to NSF grant AST-1009987), and C.J.G (for a NASA grant). E.O.O is incumbent of the Arye Dissentshik career development chair and is grateful to support by a grant from the Israeli Ministry of Science and the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (grant No 1829/12). We thank the staff at the Palomar Hale telescope for help and support with observations. We thank the anonymous referee for the careful reading of our manuscript and the valuable comments. This article is based on observations obtained with the Samuel Oschin Telescope as part of the Palomar Transient Factory project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. It is also partially based on observations obtained as part of the Intermediate Palomar Transient Factory project, a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Millwakee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, the Kavli Institute for the Physics and Mathematics of the Universe, and the Inter-University Centre for Astronomy and Astrophysics.

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Published - 0004-637X_776_1_26.pdf

Submitted - 1308.0857v1.pdf


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