Chemical abundances in the ancient Milky Way: G-type SDSS stars. Automated determination of T_{eff}, log g, [Fe/H] and [alpha /F]

We have developed TGMETalpha in order to determine T_{eff}, log g, [Fe/H] and [alpha /Fe] for large samples of FGK stars observed at various spectral resolutions. Tests on several hundred echelle spectra of reference stars degraded to low resolution (R=1.000) indicate typical rms precisions of sigma (T_{eff})˜150 K, sigma (logg)˜0.44, sigma ([Fe/H])˜0.15 and sigma ([alpha /Fe])˜0.06. We have used TGMETalpha to determine atmospheric parameters and alpha -abundances from ˜15000 SDSS stellar spectra. Thanks to this very large sample we have investigated the vertical, radial and rotational properties as well as the abundance ratio of alpha elements to iron, [alpha /Fe], of samples representative of the thick disk and the halo. Results presented in the poster, provide new constraints on galactic thick disk formation models. We find evidence for a strong negative radial gradient in [alpha /Fe] with a lack or small one in [Fe/H] in the thick disk. Moreover, there is a lack of vertical gradient in both [Fe/H] and [alpha /Fe]. These features have to be reconciled with the timescale of the thick disk formation as well as with the SFR, IMF, infall of gas or accreted satellite debris.