Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars
Detailed abundance analysis of OGLE-2008-BLG-209ST. Bensby1, J. A. Johnson2, J. Cohen3, S. Feltzing4, A. Udalski5, A. Gould2, W. Huang6, I. Thompson7, J. Simmerer4, and D. Adén4
1 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
2 Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
3 Palomar Observatory, Mail Stop 105-24, California Institute of Technology, Pasadena, CA 91125, USA
4 Lund Observatory, Box 43, 221 00 Lund, Sweden
5 Warsaw University Observatory, A1. Ujazdowskie 4, 00-478 Warszawa, Poland
6 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
7 The Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101, USA
Received 8 January 2009 / Accepted 17 March 2009
Aims. Our aims are twofold. First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge. Contrary to the cool giant stars, with their extremely crowded spectra, the dwarf stars are hotter, their spectra are cleaner, and the elemental abundances of the atmospheres of dwarf and subgiant stars are largely untouched by the internal nuclear processes of the star.
Methods. We present a detailed elemental abundance analysis of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the Bulge, for which we obtained a high-resolution spectrum with the MIKE spectrograph on the Magellan Clay telescope. We have performed four different analyses of OGLE-2008-BLG-209S. One method is identical to the one used for a large comparison sample of F and G dwarf stars, mainly thin and thick disc stars, in the Solar neighbourhood. We have also re-analysed three previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and OGLE-2007-BLG-349S with the same method. This homogeneous data set, although small, enables a direct comparison between the different stellar populations.
Results. We find that OGLE-2008-BLG-209S is a subgiant star that has a metallicity of [Fe/H] -0.33. It possesses [/Fe] enhancements similar to what is found for Bulge giant stars at the same metallicity, and what also is found for nearby thick disc stars at the same metallicity. In contrast, the previous three microlensing dwarf stars have very high metallicities, [Fe/H] +0.4, and more solar-like abundance ratios, i.e. [/Fe] 0. The decrease in the [/Fe] ratio with [Fe/H] is the typical signature of enrichment from low and intermediate mass stars. We furthermore find that the results for the four Bulge stars, in combination with results from studies of giant stars in the Bulge, seem to favour a secular formation scenario for the Bulge.
Key words: gravitational lensing -- Galaxy: bulge -- Galaxy: formation -- Galaxy: evolution -- stars: fundamental parameters -- stars: abundances
© ESO 2009