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

We report detailed chemical abundance analysis of 27 RGB stars towards theGalactic bulge in Baade's Window for elements produced by massive stars: O, Na,Mg, Al, Si, Ca and Ti. All of these elements are overabundant in the bulgerelative to the disk, especially Mg, indicating that the bulge is enhanced inType~II supernova ejecta and most likely formed more rapidly than the disk. Weattribute a rapid decline of [O/Fe] to metallicity-dependent yields of oxygenin massive stars, perhaps connected to the Wolf-Reyet phenomenon. he explosivenucleosynthesis alphas, Si, Ca and Ti, possess identical trends with [Fe/H],consistent with their putative common origin. We note that different behaviorsof hydrostatic and explosive alpha elements can be seen in the stellarabundances of stars in Local Group dwarf galaxies. We also attribute thedecline of Si,Ca and Ti relative to Mg, to metallicity- dependent yields forthe explosive alpha elements from Type~II supernovae. The starkly smallerscatter of [/Fe] with [Fe/H] in the bulge, as compared to the halo, isconsistent with expected efficient mixing for the bulge. The metal-poor bulge[/Fe] ratios are higher than ~80% of the halo. If the bulge formed fromhalo gas, the event occured before ~80% of the present-day halo was formed. Thelack of overlap between the thick and thin disk composition with the bulge doesnot support the idea that the bulge was built by a thickening of the diskdriven by the bar. The trend of [Al/Fe] is very sensitive to the chemicalevolution environment. A comparison of the bulge, disk and Sgr dSph galaxyshows a range of ~0.7 dex in [Al/Fe] at a given [Fe/H], presumably due to arange of Type~II/Type~Ia supernova ratios in these systems.Comment: 51 pages, 6 tables, 27 figures, submitte