Abundances of Baade's Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd Elements

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.

Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible by the generous financial support of the W. M. Keck Foundation.