Evidence for the Third Stellar Population in the Milky Way's Disk
  The Milky Way is a unique laboratory, where stellar properties can be
measured and analyzed in detail. In particular, stars in the older populations
encode information on the mechanisms that led to the formation of our Galaxy.
In this article, we analyze the kinematics, spatial distribution, and chemistry
of a large number of stars in the Solar Neighborhood, where all of the main
Galactic components are well-represented. We find that the thick disk comprises
two distinct and overlapping stellar populations, with different kinematic
properties and chemical compositions. The metal-weak thick disk (MWTD) contains
two times less metal content than the canonical thick disk, and exhibits
enrichment of light elements typical of the oldest stellar populations of the
Galaxy. The rotational velocity of the MWTD around the Galactic center is ~ 150
km s^(-1), corresponding to a rotational lag of 30 km s^(-1) relative to the
canonical thick disk (~ 180 km s^(-1)), with a velocity dispersion of 60 km
s^(-1). This stellar population likely originated from the merger of a dwarf
galaxy during the early phases of our Galaxy's assembly, or it is a precursor
disk, formed in the inner Galaxy and brought into the Solar Neighborhood by bar
instability or spiral-arm formation mechanisms.
