Chemo-orbital evidence from SDSS/SEGUE G-type dwarf stars for a mixed
  origin of the Milky Way's thick disk
  We combine the estimated metallicities [Fe/H], abundances [\alpha/Fe],
positions and motions of a sample of 27,500 local (7<R/kpc<9, 0.5<|z|/kpc<2.5)
SDSS/SEGUE G-type dwarf stars to investigate the chemo-orbital properties of
the Milky Way's disk around the Sun. When we derive the orbital properties
reflecting angular momentum, circularity, and thickness as function of
[\alpha/Fe] vs. [Fe/H], we find that there is a smooth variation with
[\alpha/Fe], a proxy for age. At the same time, the orbital properties of the
old stars with [\alpha/Fe]$\gtrsim$0.25 do show a transition with [Fe/H]: below
[Fe/H]$\simeq$-0.6 the orbital angular momentum decreases, and the orbits
become significantly non-circular and thicker. Radial migration of stars into
the Solar neighborhood would naturally result in a smooth variation in the
orbital properties, but the latter old metal-poor stars form a clear challenge,
in particular because a basic feature of radial migration is that stars remain
on near-circular orbits. When we next select stars on near-circular orbits, we
indeed find besides the \alpha-young 'thin-disk' stars a significant
contribution to the \alpha-old 'thick-disk' metal-rich stars. However, the
remaining \alpha-old 'thick-disk' stars on eccentric orbits, including nearly
all old metal-poor stars, are difficult to explain with radial migration alone,
but might have formed through early-on gas-rich mergers. We thus find
chemo-orbital evidence that the thicker component of the Milky Way disk is not
distinct from the thin component as expected from smooth internal evolution
through radial migration, except for the old metal-poor stars with different
orbital properties which could be part of a distinct thick-disk component
formed through an external mechanism.
