AAS 201st Meeting, January, 2003
Session 50. The Milky Way: From Center to Halo
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[50.13] Too Few Binaries on Retrograde Galactic Orbits

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
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