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 the Galactic 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 bulge relative to the disk, indicating that the bulge is enhanced in Type II supernova ejecta and most likely formed more rapidly than the disk. Our [Mg/Fe] ratios, confirmed by [Al/Mg], declines much more slowly with [Fe/H] than O, Si, Ca and Ti. The [Mg/Fe] ratio stays above +0.25 dex up to well above solar metallicity. We attribute the rapid decline of [O/Fe] to a metallicity-dependent modulation of the oxygen yield from massive stars, perhaps connected to the Wolf-Reyet phenomenon. The explosive nucleosynthesis alphas, Si, Ca and Ti, relative to Fe, possess identical trends with [Fe/H], consistent with their putative common origin. We note that different behaviors of hydrostatic and explosive alpha elements can be seen in the stellar abundances of stars in Local Group dwarf galaxies. We also attribute the decline of Si,Ca and Ti, relative to Mg, to metallicity-dependent yields for these explosive alpha elements from Type II supernovae. The production of explosive alphas in Type Ia supernovae likely explains the absence of obvious difference between Mg and Si, Ca, Ti trends in the Galactic thin disk. An alternative explanation for the [Mg/] increase with metallicity is an excess population of 30–35M⊙ stars in the bulge that grew in importance with [Fe/H]. In this model the bulge formation timescale would have been significantly longer than our favored scenario of declining metallicitydependent yields from Type II supernovae. The starkly smaller scatter of [/Fe] with [Fe/H] in the bulge, as compared to the halo, is consistent with expected efficient mixing for the bulge. Since the metal-poor bulge [/Fe] ratios are higher than ∼80% of the halo, the early bulge could not have formed from gas with the present-day halo composition. If the bulge formed from halo gas, it occurred before ∼80% of today’s stellar halo; and the halo subsequently reduced its alpha/Fe ratios significantly. The lack of overlap between the thick and thin disk composition with the bulge does not support the idea that the bulge was built by a thickening of the disk driven by the bar. The trend of [Al/Fe] is very sensitive to the chemical evolution environment: a comparison of the bulge, disk and Sagittarius dwarf spheroidal galaxy shows a range of ∼0.7 dex in [Al/Fe] at a given [Fe/H]; presumably due to a range of Type II/Type Ia supernova ratios in these systems. Subject headings: star: abundances