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We present a detailed elemental abundance study of 90 F and G dwarf, turn-offand subgiant stars in the Galactic bulge. Based on high-resolution spectraacquired during gravitational microlensing events, stellar ages and abundancesfor 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have beendetermined. We find that the Galactic bulge has a wide metallicity distributionwith significant peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41. We alsofind a high fraction of intermediate-age to young stars: at [Fe/H]>0 more than35 % are younger than 8 Gyr. For [Fe/H]<-0.5 most stars are 10 Gyr or older. Wehave also identified several episodes when significant star formation in thebulge happened: 3, 6, 8, and 12 Gyr ago. We further find that the "knee" in thealpha-element abundance trends of the sub-solar metallicity bulge is located atabout 0.1 dex higher [Fe/H] than in the local thick disk. The Galactic bulgehas complex age and abundance properties that appear to be tightly connected tothe main Galactic stellar populations. In particular, the peaks in themetallicity distribution, the star formation episodes, and the abundancetrends, show similarities with the properties of the Galactic thin and thickdisks. At the same time there are additional components not seen outside thebulge region, and that most likely can be associated with the Galactic bar. Forinstance, the star formation rate appears to have been slightly faster in thebulge than in the local thick disk, which most likely is an indication of thedenser stellar environment closer to the Galactic centre. Our resultsstrengthen the observational evidence that support the idea of a secular originfor the Galactic bulge, formed out of the other main Galactic stellarpopulations present in the central regions of our Galaxy.

astronomy and astrophysics

Chemical evolution of the Galactic bulge as traced by microlensed dwarf  and subgiant stars. VI. Age and abundance structure of the stellar  populations in the central sub-kpc of the Milky Way