Chemical evolution of the Galactic bulge as traced by microlensed dwarf
  and subgiant stars. IV. Two bulge populations
  [ABRIDGED] Based on high-resolution (R~42000 to 48000) and high
signal-to-noise (S/N~50 to 150) spectra obtained with UVES/VLT, we present
detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y,
and Ba) and stellar ages for 26 microlensed dwarf and subgiant stars in the
Galactic bulge. The analysis is based on equivalent width measurements and
standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li
abundances based on line synthesis of the 7Li line at 670.8 nm. We show that
the bulge metallicity distribution (MDF) is double-peaked; one peak at [Fe/H]=
-0.6 and one at [Fe/H]=+0.3, and with a dearth of stars around solar
metallicity. This is in contrast to the MDF derived from red giants in Baade's
window, which peaks at this exact value. A simple significance test shows that
it is extremely unlikely to have such a gap in the microlensed dwarf star MDF
if the dwarf stars are drawn from the giant star MDF. To resolve this issue we
discuss several possibilities, but we can not settle on a conclusive solution
for the observed differences. We further find that the metal-poor bulge dwarf
stars are predominantly old with ages greater than 10\,Gyr, while the
metal-rich bulge dwarf stars show a wide range of ages. The metal-poor bulge
sample is very similar to the Galactic thick disk in terms of average
metallicity, elemental abundance trends, and stellar ages. Speculatively, the
metal-rich bulge population might be the manifestation of the inner thin disk.
If so, the two bulge populations could support the recent findings, based on
kinematics, that there are no signatures of a classical bulge and that the
Milky Way is a pure-disk galaxy. Also, recent claims of a flat IMF in the bulge
based on the MDF of giant stars may have to be revised based on the MDF and
abundance trends probed by our microlensed dwarf stars.
