Observational Properties of the Metal-Poor Thick Disk of the Milky Way
  Galaxy and Insights into Its Origins
  We have undertaken the study of the elemental abundances and kinematic
properties of a metal-poor sample of candidate thick-disk stars selected from
the RAVE spectroscopic survey of bright stars to differentiate among the
present scenarios of the formation of the thick disk. In this paper, we report
on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74
main-sequence/sub-giant branch metal-poor stars, which serves to augment our
previous sample of only giant stars. We find that the thick disk [alpha/Fe]
ratios are enhanced, and have little variation (<0.1 dex), in agreement with
our previous study. The augmented sample further allows, for the first time,
investigation of the gradients in the metal-poor thick disk. For stars with
[Fe/H] < -1.2, the thick disk shows very small gradients, <0.03 +/- 0.02
dex/kpc, in alpha-enhancement, while we find a +0.01 +/- 0.04 dex/kpc radial
gradient and a -0.09 +/- 0.05 dex/kpc vertical gradient in iron abundance. In
addition, we show that the peak of the distribution of orbital eccentricities
for our sample agrees better with models in which the stars that comprise the
thick disk were formed primarily in the Galaxy, with direct accretion of stars
contributing little. Our results thus disfavor direct accretion of stars from
dwarf galaxies into the thick disk as a major contributor to the thick disk
population, but cannot discriminate between alternative models for the thick
disk, such as those that invoke high-redshift (gas-rich) mergers, heating of a
pre-existing thin stellar disk by a minor merger, or efficient radial migration
of stars.
