The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

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We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F BSS/BHB, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for F BSS/BHB is 4.0 in the thick disk (at 3 kpc < |Z| < 4 kpc), declining to on the order of 1.5-2.0 in the inner-halo region; this ratio continues to decline to 1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5% of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.