The Chemical Composition of the Galactic Bulge and Implications for its
  Evolution
  The average bulge [Fe/H] and [Mg/H] are +0.06 and +0.17 dex, respectively, in
Baade's Window, roughly 0.2 dex higher than the thin disk and ~0.7 dex higher
than the local thick disk metallicity. This suggests a higher effective yield
in the bulge, perhaps due to more efficient retention of supernova ejecta. The
bulge vertical [Fe/H] gradient, at ~0.5 dex/kpc, appears to be due to a
changing mixture of sub-populations (near +0.3 dex and -0.3 dex and one
possibly near -0.7 dex) with latitude. The bulge is enhanced in O, Mg, Si, Ca,
Ti, and Al relative to the sun, with [alpha/Fe]=+0.15 dex at [Fe/H]=0.0 dex.
Below [Fe/H]~-0.5 dex, the bulge and local thick disk compositions are very
similar, but small [Mg/Fe] and possibly [<SiCaTi>/Fe] enhancements, low [La/Eu]
ratios and large [Cu/Fe], relative to the thick disk suggest slightly higher
SFR in the bulge. However, these composition differences could simply be due to
measurement errors and non-LTE effects. Unfortunately, comparison with the
thick disk near solar [Fe/H] suffers considerable confusion, due to poor
identification of the local thick disk. Unusual zig-zag abundance trends of
[Cu/Fe] and [Na/Fe] suggest nucleosynthesis dominated by core-collapse
supernovae with metallicity-dependent yields, in the Type~Ia supernova
time-delay scenario. Thus, the bulge sub-population compositions resemble the
local thin and thick disks, but at higher [Fe/H], suggesting a radial [Fe/H]
gradient in both the thin and thick disks, within the solar circle, near -0.04
to -0.05 dex/kpc. If the bulge was built through accretion of inner disk stars
by a bar, it appears that the inner thin and thick disk stars, incorporated
into the bulge, retained vertical scale heights characteristic of their
kinematic origin, and resulting in the vertical [Fe/H] gradient and [alpha/Fe]
trends seen today.
