2007ApJ...661.1152F

We report detailed abundances of O, Na, Mg, Al, Si, Ca, and Ti–elements produced by massive stars–for 27 red giants toward the Galactic bulge in Baade's window. These species are overabundant in the bulge relative to the disk, consistent with enhancement by Type II SN ejecta. [Mg/Fe]=+0.3 dex over the full range of [Fe/H], while O, Si, Ca, and Ti are elevated but follow more disklike trends. We propose that the decline in [O/Fe] is due to metallicity-dependent oxygen yields from massive stars, perhaps connected with the Wolf-Rayet phenomenon. The elements Si, Ca, and Ti, believed to be produced during explosive nucleosynthesis, possess identical trends with [Fe/H]. We attribute the decline of these elements to metallicity-dependent yields in Type II SNe. The trend of [Al/Fe] is found to vary strikingly with environment; the range from the Sgr dwarf to the bulge is 0.7 dex. The disjoint composition of the thick/thin disk and bulge stars is inconsistent with models in which the bulge formed from the thickening of the disk, while the elevated alpha elements are consistent with a rapid bulge formation timescale. The starkly smaller scatter of [<SiCaTi>/Fe] with [Fe/H] in the bulge compared with the halo is consistent with the expectation that the bulge should have efficiently mixed. The metal-poor bulge [<SiCaTi>/Fe] ratios are higher than ∼80% of the halo; the bulge could not have formed from gas with the present-day halo composition.