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

IV. Two bulge populations

¹ Lund ObservatoryDepartment of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
e-mail: tbensby@astro.lu.se
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³ Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, São Paulo, 05508-900, SP Brasil
⁴ Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
⁵ Max Planck Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
⁶ INAF-Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁸ Palomar Observatory, Mail Stop 249-17, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Institute of Information and Mathematical Sciences, Massey University, Albany Campus, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
¹⁰ Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
¹¹ Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
¹² Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
¹³ Solar-Terrestrial Enivironment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, 464-8601 Nagoya, Japan
¹⁴ Tokyo Metropolitan College of Industrial Technology, Tokyo 116-8523, Japan
¹⁵ Department of Physics, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
¹⁶ Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

Received: 11 April 2011
Accepted: 26 July 2011

Abstract

Based on high-resolution (R ≈ 42   000 to 48   000) and high signal-to-noise (S/N ≈ 50 to 150) spectra obtained with UVES/VLT, we present detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba) and stellar ages for 12 new microlensed dwarf and subgiant stars in the Galactic bulge. Including previous microlensing events, the sample of homogeneously analysed bulge dwarfs has now grown to 26. The analysis is based on equivalent width measurements and standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li abundances based on line synthesis of the ⁷Li line at 670.8 nm. The results from the 26 microlensed dwarf and subgiant stars show that the bulge metallicity distribution (MDF) is double-peaked; one peak at  [Fe/H]  ≈ −0.6 and one at  [Fe/H]  ≈  + 0.3, and with a dearth of stars around solar metallicity. This is in contrast to the MDF derived from red giants in Baade’s window, which peaks at this exact value. A simple significance test shows that it is extremely unlikely to have such a gap in the microlensed dwarf star MDF if the dwarf stars are drawn from the giant star MDF. To resolve this issue we discuss several possibilities, but we can not settle on a conclusive solution for the observed differences. We further find that the metal-poor bulge dwarf stars arepredominantly old with ages greater than 10 Gyr, while the metal-rich bulge dwarf stars show a wide range of ages. The metal-poor bulge sample is very similar to the Galactic thick disk in terms of average metallicity, elemental abundance trends, and stellar ages. Speculatively, the metal-rich bulge population might be the manifestation of the inner thin disk. If so, the two bulge populations could support the recent findings, based on kinematics, that there are no signatures of a classical bulge and that the Milky Way is a pure-disk galaxy. Also, recent claims of a flat IMF in the bulge based on the MDF of giant stars may have to be revised based on the MDF and abundance trends probed by our microlensed dwarf stars.