We seek to constrain the formation of the Galactic bulge by means of analysing the detailed chemical composition of a large sample of red clump stars in Baades window. We measure [Fe/H] in a sample of 219 bulge red clump stars from R=20000 resolution
spectra obtained with FLAMES/GIRAFFE at the VLT, using an automatic procedure, differentially to the metal-rich local reference star muLeo. For a subsample of 162 stars, we also derive [Mg/H] from spectral synthesis around the MgI triplet at 6319A. The Fe and Mg metallicity distributions are both asymmetric, with median values of +0.16 and +0.21 respectively. The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred around [Fe/H]=-0.30 and [Mg/H]=-0.06 with a large dispersion and a narrow metal-rich component centred around [Fe/H]=+0.32 and [Mg/H]=+0.35. The metal poor component shows high [Mg/Fe] ratios (around 0.3) whereas stars in the metal rich component are found to have near solar ratios. Babusiaux et al. (2010) also find kinematical differences between the two components: the metal poor component shows kinematics compatible with an old spheroid whereas the metal rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarios for the two populations: a rapid formation timescale as an old spheroid for the metal poor component (old bulge) and for the metal rich component, a formation over a longer time scale driven by the evolution of the bar (pseudo-bulge).
We determine the iron distribution function (IDF) for bulge field stars, in three different fields along the Galactic minor axis and at latitudes b=-4 deg, b=-6 deg, and b=-12 deg. A fourth field including NGC6553 is also included in the discussion.
About 800 bulge field K giants were observed with the GIRAFFE spectrograph of FLAMES@VLT at spectral resolution R~20,000. Several of them were observed again with UVES at R~45,000 to insure the accuracy of the measurements. The LTE abundance analysis yielded stellar parameters and iron abundances that allowed us to construct an IDF for the bulge that, for the first time, is based on high-resolution spectroscopy for each individual star. The IDF derived here is centered on solar metallicity, and extends from [Fe/H]~ -1.5 to [Fe/H]~ +0.5. The distribution is asymmetric, with a sharper cutoff on the high-metallicity side, and it is narrower than previously measured. A variation in the mean metallicity along the bulge minor axis is clearly between b=-4 deg and b=-6 deg ([Fe/H] decreasing by ~ 0.6 dex per kpc). The field at b=-12 deg is consistent with the presence of a gradient, but its quantification is complicated by the higher disk/bulge fraction in this field. Our findings support a scenario in which both infall and outflow were important during the bulge formation, and then suggest the presence of a radial gradient, which poses some challenges to the scenario in which the bulge would result solely from the vertical heating of the bar.
