Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars ^*,**

II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile e-mail: tbensby@eso.org
² Lund Observatory, Box 43, 221 00 Lund, Sweden
³ Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
⁴ Max Planck Institute for Astrophysik, Garching, Germany
⁵ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁷ INAF-Astronomical Observatory of Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁸ Universiteit van Amsterdam, Sterrenkundig Instituut “Anton Pannekoek”, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁹ Solar-Terrestrial Enivironment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
¹⁰ Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
¹¹ Institute of Information and Mathematical Sciences, Massey University, Albany Campus, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
¹² Warsaw University Observatory, A1. Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 26 November 2009
Accepted: 6 January 2010

Abstract

Context. The Bulge is the least understood major stellar population of the Milky Way. Most of what we know about the formation and evolution of the Bulge comes from bright giant stars. The underlying assumption that giants represent all the stars, and accurately trace the chemical evolution of a stellar population, is under debate. In particular, recent observations of a few microlensed dwarf stars give a very different picture of the evolution of the Bulge from that given by the giant stars.

Aims. We aim to resolve the apparent discrepancy between Bulge metallicity distributions derived from microlensed dwarf stars and giant stars. Additionally, we aim to put observational constraints on the elemental abundance trends and chemical evolution of the Bulge.

Methods. We perform a detailed elemental abundance analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra that were obtained while the stars were optically magnified during gravitational microlensing events. The analysis method is the same as for a large sample of F and G dwarf stars in the Solar neighbourhood, enabling a fully differential comparison between the Bulge and the local stellar populations in the Galactic disc.

Results. We present detailed elemental abundances and stellar ages for six new dwarf stars in the Galactic bulge. Combining these with previous events, here re-analysed with the same methods, we study a homogeneous sample of 15 stars, which constitute the largest sample to date of microlensed dwarf stars in the Galactic bulge. We find that the stars span the full range of metallicities from [Fe/H] = -0.72 to +0.54, and an average metallicity of [Fe/H] = -0.08 ± 0.47, close to the average metallicity based on giant stars in the Bulge. Furthermore, the stars follow well-defined abundance trends, that for [Fe/H]<0 are very similar to those of the local Galactic thick disc. This suggests that the Bulge and the thick disc have had, at least partially, comparable chemical histories. At sub-solar metallicities we find the Bulge dwarf stars to have consistently old ages, while at super-solar metallicities we find a wide range of ages. Using the new age and abundance results from the microlensed dwarf stars we investigate possible formation scenarios for the Bulge.