The GIRAFFE Inner Bulge Survey (GIBS)

II. Metallicity distributions and alpha element abundances at fixed Galactic latitude ^⋆,⋆⋆

¹ European Southern Observatory, A. de Cordova 3107, 19001 Casilla Santiago 19, Chile
e-mail: ogonzale@eso.org
² Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 4860 Av. Vicuña Mackenna, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ Laboratoire Lagrange UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 04, France
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁶ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁷ INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ GEPI, Observatoire de Paris, CNRS UMR 8111, Université Paris Diderot, 92125 Meudon Cedex, France
⁹ Departamento de Ciencias Físicas, Universidad Andrés Bello, 220 República, Santiago, Chile
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Vatican Observatory, V 00120 Vatican City State, Italy

Received: 14 June 2015
Accepted: 3 August 2015

Abstract

Aims. We investigate metallicity and α-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° ≲ b ≲ −3.4° and −10° ≲ l ≲ +10°, were obtained within the GIRAFFE Inner Bulge Survey (GIBS) project. To this sample we added another ~400 stars in Baade’s 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 ⟨ [Fe/H] ⟩ = −0.31 dex and σ = 0.31 dex. The metal-rich component is narrower, with mean ⟨ [Fe/H] ⟩ = + 0.26 and σ = 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 α-element abundance distributions between different fields.