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B. W. Carney (UNC), D. W. Latham (CfA), J. B. Laird (Bowling Green State Univ.), L. Aguilar (UNAM)
We report new results regarding the frequency of binary systems among metal-poor field stars as a function of their kinematics. The frequency of spectroscopic binary stars among metal-poor stars is very similar to that of local metal-rich dwarfs (Latham et al.\ 2002; Goldberg et al.\ 2002). The spectroscopic binary frequency does not appear to be related to the stellar velocities perpendicular to the plane (|W| velocity) or in the Galactic radial direction (|U| velocity). However, there is a strong relationship between binary frequency, including only spectroscopic binaries, or adding in visual binaries and common proper motion pairs, and the Galactic V velocity. The 653 stars in our program with [Fe/H] \leq\ -1.0 have a binary frequency of 27.1 ±1.3% for stars on prograde orbits, but only 10.5 ±1.7% for stars on retrograde orbits. The distribution of orbital periods is similar for prograde and retrograde samples. The simplest explanation is that our sample includes a significant number of stars from an object captured by our Galaxy on a retrograde orbit that was deficient in binaries, perhaps only among its least strongly bound outer stars. Such an object could have been a dynamically evolved globular cluster or a dwarf galaxy that formed that way, such as Hurley-Keller, Mateo, & Grebel (1999) appear to have found to be the case for the Sculptor dwarf galaxy.
We thank NSF for financial support to UNC and BGSU for this work.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.