Signatures of SN Ia in the galactic thick disk Observational evidence from (alpha)-elements in 67 dwarf stars in the solar neighbourhood

We present the first results of a larger study into the stellar abundances and chemical trends in long-lived dwarf stars in the solar neighbourhood that belong to (based on their kinematics) the thin and thick galactic disk, respectively. We confirm that the trends of (alpha)-elements in the thin and thick disk are distinct (this has previously been shown for Mg by Fuhrmann 1998, but e.g. Chen et al. 2000 claimed the trends to follow smoothly upon each other). We find that the thick disk show the typical signature of contribution from SN Ia (i.e. the "knee") to the enrichment of the interstellar gas out of which the later generations of thick disk stars formed. The trend starts out as [Mg/Fe] ~ 0.35 at [Fe/H] ~ -0.7 and continue on this level with increasing [Fe/H] until -0.4 dex where a decline in [Mg/Fe] starts and steadily continues down to 0 dex at solar metallicity. The same is true for the other (alpha)-elements (e.g. Si). Using ages from the literature we find that the thick disk in the mean is older than the thin disk. Combining our results with other observational facts we suggest that the most likely formation scenario for the thick disk is, still, a violent merger event. We also suggest that there might be tentative evidence for diffusion of orbits in todays thin disk (based on kinematics in combination with elemental abundances).