  ABRIDGED: METHODS: We have conducted a high-resolution spectroscopic study of
714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star
sample has been kinematically selected to trace the Galactic thin and thick
disks to their extremes...... The determination of stellar parameters and
elemental abundances is based on a standard 1-D LTE analysis using equivalent
width measurements in high-resolution (R=40000-110000) and high signal-to-noise
(S/N=150-300) spectra obtained with..... RESULTS: .... Our data show that there
is an old and alpha-enhanced disk population, and a younger and less
alpha-enhanced disk population. While they overlap greatly in metallicity
between -0.7<[Fe/H]<+0.1, they show a bimodal distribution in [alpha/Fe]. This
bimodality becomes even clearer if stars where stellar parameters and
abundances show larger uncertainties (Teff<5400 K) are discarded, showing that
it is important to constrain the data set to a narrow range in the stellar
parameters if small differences between stellar populations are to be revealed.
We furthermore find that the alpha-enhanced population has orbital parameters
placing the stellar birthplaces in the inner Galactic disk while the low-alpha
stars mainly come from the outer Galactic disk........... We furthermore have
discovered that a standard 1-D, LTE analysis, utilising ionisation and
excitation balance of Fe I and Fe II lines produces a flat lower main sequence.
As the exact cause for this effect is unclear we chose to apply an empirical
correction. Turn-off, and more evolved, stars, appears to be un-affected.
