  We performed a uniform and detailed analysis of 1112 high-resolution spectra
of FGK dwarfs obtained with the HARPS spectrograph at the ESO 3.6 m telescope
(La Silla, Chile). Most stars have effective temperatures 4700 K < Teff < 6300
K and lie in the metallicity range of -1.39 < [Fe/H] < 0.55. Our main goal is
to investigate whether there are any differences between the elemental
abundance trends (especially [alpha/Fe] ratio) for stars of different
subpopulations. The equivalent widths of spectral lines are automatically
measured from HARPS spectra with the ARES code. The abundances of three alpha
elements are determined using a differential LTE analysis relative to the Sun,
with the 2010 revised version of the spectral synthesis code MOOG and a grid of
Kurucz ATLAS9 atmospheres.The stars of our sample fall into two populations,
clearly separated in terms of [alpha/Fe] up to super-solar metallicities. In
turn, high-alpha stars are also separated into two families with a gap in both
[alpha/Fe] ([alpha/Fe] = 0.17) and metallicity ([Fe/H] = -0.2) distributions.
The metal-poor high-alpha stars (thick disk) and metal-rich high-alpha stars
are on average older than chemically defined thin disk stars (low-alpha stars).
The two alpha-enhanced families have different kinematics and orbital
parameters. The metal-rich alpha-enhanced stars, such as thin disk stars have
nearly circular orbits, close to the Galactic plane. We put forward the idea
that these stars may have been formed in the inner Galactic disk, but their
exact nature still remains to be clarified.
