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 stellarabundances and chemical trends in long-lived dwarf stars in the solarneighbourhood that belong to (based on their kinematics) the thin andthick galactic disk, respectively. We confirm that the trends of alpha-elements in the thin and thick disk are distinct (this has previouslybeen shown for Mg by Fuhrmann cite{Fuhrmann}, but e.g. Chen et al.cite{Chen} claimed the trends to follow smoothly upon each other). Wefind that the thick disk show the typical signature of contribution fromSN Ia (i.e. the ``knee'') to the enrichment of the interstellar gas outof which the later generations of thick disk stars formed. The trendstarts out as [Mg/Fe] ~ 0.35 at [Fe/H] ~ -0.7 and continue on this levelwith increasing [Fe/H] until -0.4 dex where a decline in [Mg/Fe] startsand steadily continues down to 0 dex at solar metallicity. The same istrue for the other alpha -elements (e.g. Si). Using ages from theliterature we find that the thick disk in the mean is older than thethin disk. Combining our results with other observational facts wesuggest that the most likely formation scenario for the thick disk is,still, a violent merger event. We also suggest that there might betentative evidence for diffusion of orbits in todays thin disk (based onkinematics in combination with elemental abundances). Based onobservations collected at the European Southern Observatory, La Silla,Chile, Proposals #65.L-0019(B) and 67.B-0108(B).