Very Metal-Poor Outer-Halo Stars with Round Orbits
  The orbital motions of halo stars in the Milky Way reflect the orbital
motions of the progenitor systems in which they formed, making it possible to
trace the mass-assembly history of the Galaxy. Direct measurement of
three-dimensional velocities, based on accurate proper motions and
line-of-sight velocities, has revealed that the majority of halo stars in the
inner-halo region move on eccentric orbits. However, our understanding of the
motions of distant, in-situ halo-star samples is still limited, due to the lack
of accurate proper motions for these stars. Here we explore a model-independent
analysis of the line-of-sight velocities and spatial distribution of a recent
sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars
within 30 kpc of the Galactic center. We find that the mean rotational velocity
of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that
of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that
the relatively more metal-rich BHB stars are dominated by stars with eccentric
orbits, as previously observed for other stellar samples in the inner-halo
region. By contrast, the very metal-poor BHB stars are dominated by stars on
rounder, lower-eccentricity orbits. Our results indicate that the motion of the
progenitor systems of the Milky Way that contributed to the stellar populations
found within 30 kpc correlates directly with their metal abundance, which may
be related to their physical properties such as gas fractions. These results
are consistent with the existence of an inner/outer halo structure for the halo
system, as advocated by Carollo et al. (2010).
