Origins of the Thick Disk as Traced by the Alpha-Elements of Metal-Poor
  Giant Stars Selected from RAVE
  Theories of thick disk formation can be differentiated by measurements of
stellar elemental abundances. We have undertaken a study of metal-poor stars
selected from the RAVE spectroscopic survey of bright stars to establish
whether or not there is a significant population of metal-poor thick-disk stars
([Fe/H] <~ -1.0) and to measure their elemental abundances. In this paper, we
present abundances of four alpha-elements (Mg, Si, Ca, Ti) and iron for a
subsample of 212 RGB and 31 RC/HB stars from this study. We find that the
[alpha/Fe] ratios are enhanced implying that enrichment proceeded by purely
core-collapse supernovae. This requires that star formation in each star
forming region had a short duration. The relative lack of scatter in the
[alpha/Fe] ratios implies good mixing in the ISM prior to star formation. In
addition, the ratios resemble that of the halo, indicating that the halo and
thick disk share a similar massive star IMF. We conclude that the alpha
enhancement of the metal-poor thick disk implies that direct accretion of stars
from dwarf galaxies similar to surviving dwarf galaxies today did not play a
major role in the formation of the thick disk.
