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

VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way^⋆,⋆⋆

¹ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
e-mail: tbensby@astro.lu.se
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ Korea Astronomy and Space Science Institute Institute, 305-348 Daejon, Republic of Korea
⁴ Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
⁵ Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138, USA
⁶ Research School of Astronomy & Astrophysics, Australian National University, Cotter Road, Canberra, ACT 2611, Australia
⁷ Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, SP, Brasil
⁸ INAF-Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹⁰ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹² Institute for Space-Earth Environmental Research, Nagoya University, 464-8601 Nagoya, Japan
¹³ Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
¹⁴ Institute for Information and Mathematical Sciences, Massey University, 1330 Auckland, New Zealand
¹⁵ Laboratory for Exoplanets and Stellar Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁶ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, 560-0043 Osaka, Japan
¹⁷ Mt. John University Observatory, PO Box 56, 8770 Lake Tekapo, New Zealand

Received: 3 February 2017
Accepted: 7 July 2017

Abstract

We present a detailed elemental abundance study of 90 F and G dwarf, turn-off, and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. Four main findings are presented: (1) a wide metallicity distribution with distinct peaks at [Fe/H] = −1.09, −0.63, −0.20, + 0.12, + 0.41; (2) ahigh fraction of intermediate-age to young stars where at [Fe/H] > 0 more than 35% are younger than 8 Gyr, and for [Fe/H] ≲ −0.5 most stars are 10 Gyr or older; (3) several episodes of significant star formation in the bulge has been identified: 3, 6, 8, and 11 Gyr ago; (4) tentatively the “knee” in the α-element abundance trends of the sub-solar metallicity bulge is located at a slightly higher [Fe/H] than in the local thick disk. These findings show that the Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. There are also additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy. Additionally, our analysis of this enlarged sample suggests that the (V−I)₀ colour of the bulge red clump should be revised to 1.09.