Based on high-resolution ( R \approx 42 000 to 48 000 ) and high signal-to-noise ( S / N \approx 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 12 new microlensed dwarf and subgiant stars in the Galactic bulge .
Including previous microlensing events , the sample of homogeneously analysed bulge dwarfs has now grown to 26 .
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 ^ { 7 } Li line at 670.8 nm .
The results from the 26 microlensed dwarf and subgiant stars show that the bulge metallicity distribution ( MDF ) is double-peaked ; one peak at [ Fe / H ] \approx - 0.6 and one at [ Fe / H ] \approx + 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 .