Two distinct halo populations in the solar neighborhood. III. Evidence
  from stellar ages and orbital parameters
  In Papers I and II of this series, the existence of two distinct halo
populations of stars have been found in the solar neighborhood. Precise
relative ages and orbital parameters are determined for 67 halo and 16
thick-disk stars having metallicities in the range -1.4 < [Fe/H] < -0.4 to
better understand the context of the two halo populations in the formation and
evolution of the Galaxy. Ages are derived by comparing the positions of stars
in the logT_{eff}-log(g) diagram with isochrones from the Y^2 models
interpolated to the exact [Fe/H] and [alpha/Fe] values of each star. Possible
systematic errors in T_{eff} and log(g) are considered and corrected. With
space velocities from Paper I as initial conditions, orbital integrations have
been carried out using a detailed, observationally constrained Milky Way model
including a bar and spiral arms. The `high-alpha' halo stars have ages 2-3 Gyr
larger than the `low-alpha' ones. The orbital parameters show very distinct
differences between the `high-alpha' and `low-alpha' halo stars. The
`low-alpha' ones have r_{max}'s to 30-40 kpc, z_{max}'s to approx. 18 kpc, and
e_{max}'s clumped at values greater than 0.85, while the `high-alpha' ones,
r_{max}'s to about 16 kpc, z_{max}'s to 6-8 kpc, and e_{max} more or less
uniformly distributed over 0.4-1.0. A dual in situ-plus-accretion formation
scenario best explains the existence and characteristics of these two halo
populations, but one remaining defect is that this model is not consistent
regarding the r_{max}'s obtained for the in situ `high-alpha' component; the
predicted values are too small. It appears that omega Cen may have contributed
in a significant way to the existence of the `low-alpha' component; recent
models, including dynamical friction and tidal stripping, have produced orbital
parameters as great as those of the `low-alpha' component.
