Chemical Abundances and Milky Way Formation

Abstract

Stellar chemical element ratios have well-defined systematic trends as a function of abundance, with an excellent correlation of these trends with stellar populations defined kinematically. This is remarkable, and has significant implications for Galactic evolution. The source function, the stellar Initial Mass Function, must be nearly invariant with time, place and metallicity. Each forming star must see a well-mixed mass-averaged IMF yield, implying low star formation rates, with most star formation in at most a few regions of similar evolutionary history. These well-established results are difficult to reconcile with standard hierachical formation models, which assemble many stellar units: galaxy evolution seems to have been dominated by gas assembly, with subsequent star formation. Recent results, and some new ones, on the Galactic bulge, stellar halo and thick disk t hat justify this conclusion are presented.