Thin and Thick Galactic Disks

Studies of elemental abundances in stars belonging to the thin and the thick disk of our Galaxy are reviewed. Edvardsson et al. (1993) found strong evidence of [α/Fe] variations among F and G main sequence stars with the same [Fe/H] and interpreted these differences as due to radial gradients in the star formation rate in the Galactic disk. Several recent studies suggest, however, that the differences are mainly due to a separation in [α/Fe] between thin and thick disk stars, indicating that these populations are discrete Galactic components, as also found from several kinematical studies. Further evidence of a chemical separation between the thick and the thin disk is obtained from studies of [Mn/Fe] and the ratio between rand s-process elements. The interpretation of these new data in terms of formation scenarios and time scales for the disk and halo components of our Galaxy is discussed.