The GIRAFFE Inner Bulge Survey (GIBS). II. Metallicity distributions and alpha element abundances at fixed Galactic latitude

Abstract

Aims: We investigate metallicity and _s13_s17945_s19-element abundance gradients along a Galactic longitude strip, at latitude b ~ -4°, with the aim of providing observational constraints for the structure and origin of the Milky Way bulge. Methods: High-resolution (R ~ 22 500) spectra for 400 K giants, in four fields within -4.8° _s13_s178818_s19 b _s13_s178818_s19 -3.4° and -10° _s13_s178818_s19 l _s13_s178818_s19 _s1610°, were obtained within the GIRAFFE Inner Bulge Survey (GIBS) project. To this sample we added another ~400 stars in Baade_s14s Window at (l,b) = (1°,-4°), observed with the identical instrumental configuration: FLAMES GIRAFFE in Medusa mode with HR13 setup. All target stars lie within the red clump of the bulge colour-magnitude diagram, thus minimising contamination from the disc or halo stars. The spectroscopic stellar surface parameters were derived with an automatic method based on the GALA code, while the [Ca/Fe] and [Mg/Fe] abundances as a function of [Fe/H] were derived through a comparison with the synthetic spectra using MOOG. We constructed the metallicity distributions for the entire sample, and for each field individually, in order to investigate the presence of gradients or field-to-field variations in the shape of the distributions. Results: The metallicity distributions in the five fields are consistent with being drawn from a single parent population, indicating the absence of a gradient along the major axis of the Galactic bar. The global metallicity distribution is nicely fitted by two Gaussians. The metal-poor component is rather broad, with a mean at _s13_s1710216_s19 [Fe/H] _s13_s1710217_s19 = -0.31 dex and _s13_s17963_s19 = 0.31 dex. The metal-rich component is narrower, with mean _s13_s1710216_s19 [Fe/H] _s13_s1710217_s19 = _s16 0.26 and _s13_s17963_s19 = 0.2 dex. The [Mg/Fe] ratio follows a tight trend with [Fe/H], with enhancement with respect to solar in the metal-poor regime similar to the value observed for giant stars in the local thick disc. [Ca/Fe] abundances follow a similar trend, but with a considerably larger scatter than [Mg/Fe]. A decrease in [Mg/Fe] is observed at [Fe/H] = -0.44 dex. This knee is in agreement with our previous bulge study of K-giants along the minor axis, but is 0.1 dex lower in metallicity than the value reported for the microlensed dwarf and subgiant stars in the bulge. We found no variation in _s13_s17945_s19-element abundance distributions between different fields.